Last update: March 6, 2024

Why Do My Fish Keep Dying? 11 Reasons & How to Prevent It

So, you just bought your first goldfish…

You take it home, only to discover your little fish floating belly up a few days later.

Or, maybe this isn’t your first fish funeral. Time and time again, you find your dead fish – It’s like you have become a fish serial killer.

At this point, you might start screaming…

Why do my fish keep dying?

Discovering the reasons behind the unfortunate demise of fish in your aquarium can be perplexing, especially for beginners. Various factors contribute to this issue, ranging from stress, improper tank setup, overfeeding, to diseases, among others.

It might seem like your fish are passing away without any apparent cause. However, while it’s conceivable that some fish may already be unhealthy at the time of purchase, the likelihood is higher that the environment or care they’re receiving is playing a significant role in their decline.

In this article, we’re going to explore the primary reasons behind the loss of your aquatic friends, shedding light on a mystery that many pet owners face.

The number one killer of pet fish…

Stressed goldfish in fish bowl that is too small will mean fish keep dying

The leading cause of mortality in pet fish is stress. Just like in humans, stress is incredibly detrimental to fish, often leading to their eventual death. The impact of stress is not immediate; it can take days or weeks to manifest visibly, but once it does, the survival of the fish is greatly compromised.

Identifying stress in fish can be challenging, particularly for beginners, as fish do not express distress in ways that are easily recognizable. You might come home one day to find all your fish lifeless, leaving you to wonder what went wrong when everything seemed fine the day before.

The reality is that the cause of stress—and consequently death—could have been present for weeks, with your fish struggling to cope until they could no longer.

Stress in fish acts like a ticking time bomb, slowly eroding their health until they succumb. The absence of visible emotion or expression in fish means that their distress can go unnoticed until it’s too late.

To help your aquatic pets thrive, it’s crucial to understand and mitigate the common stressors they face.

Let’s delve into these causes and learn how to create a more conducive environment for your fish, ensuring their longevity and well-being.

1. You didn’t properly set up your aquarium

Boy performing fish-in cycle with goldfish inside tank

Setting up an aquarium is easy – grab a glass tank, fill it with water, add your dechlorinator. Now, all that’s left is to add your fish, right?

Well… No.

Believe it or not, you might not be able to add fish to your new aquarium for a few weeks.

I mean, you can, but there is a high risk that they will die soon.

The first thing you want to do is cycle your aquarium.

You see, fish are not the only things that live in your aquarium. A tank with healthy water quality also has bacteria that break down and remove waste from your aquarium.

To put it simply, this bacteria keeps the water safe for your fish.

Just one problem – this bacteria does not exist in a new aquarium. If you add your fish now, your fish can become stressed and die while it needs a stress-free environment.

So, what you want to do is introduce this bacteria into your tank water – ideally before you buy your fish.

The process of establishing this good bacteria is referred to as cycling your aquarium.

Ideally, cycling your aquarium should be done before you buy your fish because the process can take a couple of weeks. FishLab’s Fishless Cycling Guide will walk you through the steps to make regular water in your aquarium, safe for your fish.

If it’s too late and you already bought fish, then you can try a fish-in cycle. Check out our Fish-in Cycle Guide to give your fish the best chance of surviving.

Note: Fish-in cycling is incredibly stressful for fish, and it’s likely that your fish will die in the process. I highly recommend cycling your new aquarium before buying fish.

2. Your aquarium setup doesn’t suit your fish

Saltwater fish in freshwater tank

Your tank should be set up to suit the fish that you keep.

A freshwater fish will die in a saltwater aquarium. Similarly, a fish that prefers warm water will likely die in cold water.

But just because your fish isn’t dead yet doesn’t mean that your setup is right. In fact, while your fish appears to be surviving, it may be stressed beyond belief. And as we discussed earlier, stress can lead to an early death.

Take betta fish for instance. They prefer calm water, so strong water movement, say from a filter nozzle or bubbler, can stress a betta out.

A pleco, on the other hand, needs plenty of places to hide. Without a safe spot to hole up, plecos can become stressed.

Meanwhile, African cichlids need hard alkaline water, the exact opposite of cardinal tetras, who thrive in soft, acidic water.

Failing to provide an appropriate environment for your fish  and ideal temperature can lead to it dying well before its time.

Because no two fish are alike, read up on the preferences of your fish before introduction to your tank. Where one fish may thrive, another fish may become stressed and eventually die.

3. Your aquarium is too small

Fish squashed into aquarium that is too small. Your fish keep dying because of overcrowding

Too many fish in your aquarium? An overcrowded tank is essentially a death sentence for your fish for numerous reasons and maybe all you need is a larger tank.

First, you know that good bacteria I talked about in the previous section – the one that breaks down waste to keep the water healthy?

Well, it can only break down so much at a time. Generally speaking, the larger the tank (and filter), the more good bacteria there will be to break down waste.

So, the more fish you add to your aquarium, the more poop. If your fish are pooping at a faster rate than the good bacteria can break it down, then your water can soon turn toxic – killing all your fish in the process.

For beginners, this is most commonly seen when betta or goldfish are kept in an aquarium that is too small, but it can happen in an aquarium of any size.

But it’s not just overcrowding that is the issue. A small tank, without much room to move, can cause a fish to become stressed, leading to a sudden death.

Finally, its important for fish tank owners to note that overcrowding can actually cause your fish to suffocate…

Like you and me, fish need oxygen to breathe – without it, they will soon die.

The problem is that there is only so much oxygen in your aquarium. And if you have too many fish, then they will be breathing faster than the oxygen can be replaced, causing them to suffocate.

And remember, fish grow. Just because your fish is small now doesn’t mean it will stay that way forever. Your goldfish might be less than 2 inches now, but it can grow to a whopping 10 inches!

4. You are mixing incompatible fish

Incompatible fish mixed together in aquarium

Not all fish get along.

While you may have the best intentions, you may have accidentally created an underwater cage fight instead of a peaceful home for your fish.

Some fish that are commonly owned by beginners are downright aggressive – fish such as cichlids, Chinese algae eaters, blue gourami and male betta can be territorial, fight over food and even attack or bully other fish.

In the wild, your fish would be able to easily escape its bully. Unfortunately, your aquarium is much smaller, and your fish has no other choice but to flee for its life, day and night.

Being constantly chased and nipped at by a territorial fish is stressful and will likely lead to your fish dying sooner than it should.

Before you purchase any new fish for your aquarium, ask about their compatibility with the other fish to avoid aquarium fish deaths.

5. You are overfeeding your fish

Fish being overfed by hand with fish food

Another extremely common form of death is overfeeding.

It always surprises a beginner just how little food fish actually eat.

Take a single betta for instance – you know those micro-pellets they eat? Well, your betta only needs three to keep full. Any more than this and you are overfeeding.

But the reason overfeeding is bad for your fish is because it causes the water to become dangerous that may lead to water poisoning.

To put it simply, whatever goes into your fish must come out. The more you feed your fish, the more they poop. Also, uneaten food decays, creating more waste which affects the water quality.

This waste quickly fouls up your water and before long, becomes toxic, killing your fish.

Also, you want to make sure that you are feeding your fish the right food. A meat eater isn’t going to live long eating fish food made from vegetables.

6. You are not performing regular aquarium maintenance

Bad smelling fish poop in aquarium next to goldfish

You should be setting time aside every week or two to clean your aquarium. This typically includes wiping down the glass, cleaning the substrate and checking to ensure everything is in working order.

But perhaps the most important maintenance is performing a water change.

You and me? We have the luxury of keeping our home clean by throwing out the trash and flushing the toilet.

Your fish on the other hand, must swim in it. If waste continues to build up and you do nothing about it will lead to water quality issue putting your fish are at risk of dying.

It is for this reason that you should perform a weekly water change.

By removing 10-20% of the gunked up aquarium water in your tank each week and replacing it with fresh water, you’ll keep your fish happy and healthy.

7. Your aquarium encountered rapid water changes

Goldfish in aquarium that is too hot, staring at thermometer and sweating. Constant temperature changes means fish keep dying due to shock

Rapid swings in water temperature, pH or salinity can shock a fish, which soon leads to death.

It is for this reason that you should acclimate any fish that you bring home before adding them to your aquarium. There can be a large difference between the water in the bag and the water in your tank temperature.

Likewise, if you are performing a large water change in your aquarium, the temperature of the water being added should not be too different from that of your aquarium.

Any changes  or shift in water parameters that you make to your aquarium should be slow and steady.

8. You killed the beneficial bacteria in your aquarium

As I touched on earlier, after you cycle your tank, you will have good bacteria living in your aquarium. And, this bacteria breaks down waste that would otherwise make your fish sick and ultimately die.

For the most part, this beneficial bacteria lives in your filter and substrate.

If this bacteria dies, then the water in your aquarium will soon turn toxic, killing your fish.

How does this bacteria die? It’s often by mistake.

If you forget to add a dechlorinator during a water change, then you will wipe out the entire bacteria colony – chlorine is a bacteria killer (and a fish killer too!).

Similarly, if you rinse your filter with routine water or chlorinated water, the chlorine will do the same.

Another way is when replacing your tank filter or filter media. Your new filter will not contain any beneficial bacteria, which will lead to waste quickly accumulating to lethal levels.

9. Your fish experienced rough travel

Goldfish in bag on the way to its new home. Lack of oxygen means fish keep dying

Can you imagine walking outside when all of a sudden someone abducts you and sticks you in a tiny cell?

Sounds stressful, right?

Actually, it sounds like a scene straight out of a horror story! But it’s not much different than the journey many fish destined for aquariums make.

If your fish is wild-caught, then it made a journey halfway across the world just to end up in a small tank in your local fish store, which is a drastic shift.

It is possible that the fish you buy are already stressed from this long journey.

Similarly, the last leg from the fish store to your home can be a killer.

That plastic bag you bring your fish home? Well, it only has so much oxygen in it.

Also, too much shaking of the bag is definitely going to stress your fish which results in an unhealthy fish.

10. Your fish has a disease or parasite

Sick goldfish in fish bowl with thermometer and ice pack

Research shows that humans are more likely to catch a cold when under stress.[1]

Well, studies show that the same goes for fish.[2]

You see, a healthy, stress-free fish has a strong immune system – their skin, scales and slime coat are more than capable of fending off any disease or parasite.

A stressed fish, on the other hand, is more prone to illness and disease, and a sick fish ultimately ends up dying.

If you suspect your fish has a disease, you can read our articles about freshwater ich and columnaris. Or you can read about planaria, which is a pest.

Keeping your fish stress-free goes a long way toward keeping your fish alive.

11. Your fish is old

Old fish with gray beard and walking stick in aquarium

Finally, you have old age.

I’ll be honest, for a beginner aquarium owner wondering why their fish died, this is perhaps the least likely answer.

However, if your fish has had a previous owner, then it is plausible that your fish passing may simply be due to leading a full life.

After all, no fish is going to live forever, and one day your lively little fish is going to give up the ghost.

Yep, no matter how well you care for your fish… There is no escaping old age.

Just how long before your fish passes entirely depends on the breed. For bettas, 4 years is considered old, while a healthy goldfish can live for 10.


I’ll be honest – in my 30 years of fishkeeping, most instances of fish dying can be put down to the owner making a mistake or just not knowing any better.

Look, accidents happen. And, there is nothing to be ashamed of.

If you made an error that lead to your fish dying, don’t beat yourself up over it. And don’t take it as a reason to not breed another fish!

Instead, use it as a learning experience. You can now provide your next fish with a full and happy life!

Can you think of any other reasons why a fish might die? Let me know In the comments below!

Ian Sterling

Ian Sterling, founder of, began his aquarium journey over 30 years ago, driven by a deep fascination for fish and their diverse personalities. His website,, is dedicated to making fishkeeping accessible and enjoyable, offering beginner-friendly guidance, expert insights, and a community for aquarists to connect and share experiences.

Comments (178)

Hi theres something killing my fish but we carnt figure out why .
Ph is fine
Nitrates nitrites are fine
No ammonia
We just carnt figure out why they are dieing .
We have quite a high ph 7.5 which is normal for our tank
We only keep fish suitable to live in these levels
Any ideas why this is happening
We have lost 10 fish over the last 7 days 3 of which were new fish and the others we have had a while
Its like they swimming around happy one minuite and the next they are dead
Tia for any advice

Hi Joanne,

There really isn’t enough info to go off here. It could be disease, lack of oxygen, chemicals in the water, etc. You’ll have to keep investigating. Why not take a photo of your setup and ask your local fish store? They can be an incredible source of help and is why I recommend finding yourself a good one.

Recently just bought a fish tank, it’s been set up not for 3 weeks, added quick start (correct amount) and tap water conditioner (correct amount)
Nitrate perfect
Nitrite perfect
Ph right where it should be
Temperature is the same as the fish shop
Ammonia 0
Oxygen is perfect
Water change every week
I’ve had 7 in to start with and they all died in a few days and then I put 5 in the second week after leaving it running for a further week and now I’ve put in 4 more and I have one remaining ( third week )

Can you give me any advise please!

Thank you

Hi Mill,

It sounds to me like you have not allowed your aquarium to cycle. If you add fish immediately to an uncycled tank, without the proper precautions, there is a very high possibility they will die.

Also, the fact that you refer to Nitrate and Nitrite as being perfect has me worried. What are the actual readings when you perform a test?

I have been working with my local petstore to set up my tank. I have tried 3 times adding fish after they say my water tested perfect. They have died everytime within 24 hours. I am doing an African Cichlid tank. I have shells in my tank and the petstore “experts” say they love shells. Should I take them out and empty the tank and replace filter cartridges and cycle the water for a few weeks? Then add the fish without the shells? I love your blog by the way.

Hi Sheryl,

You can test the water yourself using an aquarium test kit like this one. You should have one on hand anyway, so that you can regularly check your water to make sure nothing has gone wrong. Testing your water will also provide clues as to whether or not you should keep the shells.

There shouldn’t be a need to replace the filter cartridges. I’d add some fish food to your tank and wait a few days. If it tests positive for ammonia, your tank isn’t cycled and you should perform a fishless cycle before attempting to add fish again.

Hi Ian, I just found your blog. I have a few questions, trying not to go into too much history. Basically I have a small 7 gallon tank, and the pattern is that I’ll have fish that seem really happy for months on end, and then suddenly I’ll have a die-out, although pH and ammonia levels are within healthy range. I gave up and totally dumped and recycled my tank after my first major die-out (which I think may have been due to adding a diseased fish.) Recently, I added new fish this summer (2 small goldfish). Admittedly, I should have waited longer for the tank to cycle and fishies 1&2 died. Then, once I’d established better water conditions, I added two more goldfish. I figured, they’re hardy, I’ll have better luck. So they’ve seemed really happy over the last few months. I added a couple of snails and a banana root plant. Everyone was happy, it seemed. A couple weeks ago the larger goldfish got lethargic and died. After it died the smaller goldfish started acting really weird, but still seemed physically healthy. It literally ate all of the stems of the banana plant (which it had never eaten before). But other than the weird behavior, it was energetic and seemingly fine until today I came home and bam! It’s dead. No signs of illness, lethargy, or anything. Now, granted, this little guy was literally a $.39cent feeder fish I took a chance on, but it’s weird that one day he seemed fine and now he’s gone. Again, the pH and ammonia are fine and the temperature has been consistent. I did add a couple more snails last week, and I’m wondering if they brought disease? Do I have too many snails? I like to keep them as algae eaters because I’ve been told a 7 gallon is too small for a pleco or other algae eaters. I can’t really get a bigger tank right now. I have been able to keep most of my fish for at least 4-6 months before disaster, some as long as a year, so it’s not like they all die instantly. The longest I’ve kept any fish was a female Betta I had for over a year, so I’m really not as negligent as it might sound. I’m really trying and I don’t understand why a seemingly healthy tank suddently turns into a graveyard, even when all the typical stress factors; pH, ammonia, temperature, are seemingly normal. Any advice is appreciated. Thanks!

Hi Dana,

I’m terribly sorry to hear about your bad experience. Please don’t let it put you off the hobby.

A couple of problems stand out to me. All of these could have been a problem, or none of them, it’s really hard to tell without seeing a tank in person.

– A 7 gallon is too small for the majority of fish, especially goldfish. There are not a whole lot of fish that you can actually keep in a 7 gallon, so read up before making your next additions. A tank this size isn’t suitable for much more than a few guppies or endlers.

– Feeder fish are kept in unhealthy,cramped conditioned and typically have a short life expectancy, It’s not exactly surprising that it soon passed.

– Are you measuring nitrite, nitrate and performing regular maintenance (including gravel vacuuming and water changes?)

I would also recommend speaking to someone in your local area who has been successful in keeping fish alive. Since they are using the same water source and similar equipment, they will be in a much better position to give advice. Failing that, is there a local aquarium club or reputable independent fish store you can got to for advice?

I have my tank am out a 4 weeks. My fish have been dying 1 at a time. Ph leaver is good. Temp. Good. I had Platt, and angle fish. Are the angle fish mean

Hi Annette,

Whether angel fish and platy are compatible entirely depends on the size of your tank. I have seen plenty of tanks with them mixed and no issues. I would assume if the tank is too small, you could have problems. Unfortunately, you have not given me enough direction to identify why your fish are dying. It could be poor water parameters (ammonia, nitrite, nitrate) the tank is too small, lack of cleaning, or any number of the causes mentioned in this article.

Hi there, hoping you can help me! My fiance’ and I have been aquarium lovers for many years, all starting in COllege with my first betta, Dexter, who lived a long healthy life full of trauma. I have made a lot of mistakes in my journey of owning fish, but am determined to get it right!

I currently have a 55g freshwater tank. it has been running for several months. In the last week(ish) we have lost a female betta, a lyretail molly, and a guppy. I have another lyretail with a greenish spot on his head that looks like algae? I’m confident that I’m doing something wrong, but am struggling to determine what that might be. I did a test strip in our water and everything looked normal. The tank is due for a water change but i believe there is something larger at play in our tank. Any ideas or advice would be greatly appreciated.
Frustrated by the lack of resources for fish owners looking to take better care of their fish. The staff at our local pet store have been incredibly helpful, but they don’t know everything either. A few months ago I took a fish in, because I wasn’t sure what was wrong and they had no idea either. If you or your readers have any favorite resources to share i would be so grateful.

Hi Katie,

That’s awesome to hear that you are looking to improve your fish keeping skills, it’s a tricky hobby, especially when many big box chain pet stores don’t train their staff correctly. There is unfortunately a lot of misinformation floating around.

I’d start at the beginning with a test. I recommend using an aquarium test kit over test strips, It’s not uncommon for test strips to incorrect results, especially those combo ones that test multiple parameters at once. I personally suggest buying an API master test kit (freshwater), this will test all the basic parameters that usually cause problems – it’s what I recommend for beginners and experts alike, in terms of affordability, no other kit comes close. Just make sure you follow the instructions and compare the colors outside, in bright daylight.

About 90% of the time, problems with die offs come down to water parameters, so this will eliminate most of the problems you are dealing with.

You’ll also want to confirm your water temperature and water change schedule. Water changes don’t just remove nitrates but also replenish essential minerals. Typically a water change is done every 1 – 2 weeks along with gravel vaccing the substrate.

I have a 220 gallon tank, 2 fluval FX-6 filters, tank is at 74 degrees I have 2 18” airstones one on each side of the tank
7.0 ph, nitrate10ppm, nitrite 0ppm, ammonia 0.
Tank is 3 months old.
Only 3 fish have survived , a neon, a tetra and a cat fish, we had 4 of each on lu one of each is still alive, every other fish we added (angel, more neons, plecos, crawfish, and some other misc community fish) all die within a week. I don’t understand, I have kept fish before with out issue in smaller tanks but this has me baffled, please advise
Thanks, George

Hi George,

Diagnosing this over the internet is going to be difficult. It could be a multitude of things:

Is your temperature swinging?
Are you using enough dechlorinator?
Is there enough oxygen in the tank?
Are the fish that dye showing signs of disease?
Does your water source contain pesticides?
Are you feeding the right food?
Is there something in your tank leaching chemicals?

In mysterious cases like this, I first recommend showing an expert, say your local independent fish store, your tank set up and discussing it with them.

Failing that, I’d strip your tank right back so that it only contains the very basics (filter, heater, light, substrate, water) and see if your fish last more than a week then. If they are still dying, this will narrow down the cause to the few remaining few things in the tank, making the problem easier to identify. If this solves the problem, then something you removed from your tank could be the cause of the deaths.

Hi Ian,
Thank you for you quick response.
There is enough O2 I have 2 big airaters, temp is consistent , the tank has been up for a year, and there have been casualties all along the way! We started with piranas we had 10 all died after about 6 months, then we started with community fish, tetras, cat fish, angels etc however there are 3 of the original community fish still alive, they are the soul survivors! They eat and always look happy, and we have gone through 100 fish since then!
Can I have too much flitration? I have 2 FX6 filters rates for 600 gallons each?
Too much current?
When we started the tank we used a gravel that had quick start “pre cycled” It was called! Could that be an issue? Can I send you a pict of the tank?
George “sad fish keeper”

Hi George,

Troubleshooting problems like this is generally trial and error until you narrow it down. In fish keeping there are about a million things that could go wrong, so it’s quite difficult to help people that don’t have a similar setup.

The reason I suggested speaking with local fish keepers is that they will use the same water source and products as you, which can further narrow down problems. Same goes for stripping back the tank. Same goes for taking out everything unnecessary.

Looking at your filtration…

It certainly is excessive filtration. Also, those 2 air stones are only going to add to the water movement. I know the saying is “you can never have too much filtration” but this doesn’t take into account the current. If fish are having trouble swimming due to water movement, this could certainly stress them to the point of death. Really, you only need a single Fluval FX6 on a tank this size. However, you could try lowering the flow on both and removing the air stones – your filter outlet should be providing enough surface agitation to maximize oxygenation that the air stones are unnecessary.

On your pre-cycled gravel, was it purchased in a bag dry? Generally you would add gravel from an existing tank that is wet. It would do no harm removing your gravel too, especially since your filters will contain more than enough beneficial bacteria for a tank this size.

Hi Ian,
Well based on your suggestion I cut back the air stones to about 20% total output and cut back the filter flow by 10%
And this makes For a much calmer tank, I can’t believe how much the air stones @100% disturbed the water!
So I’m 11 days with no dead fish! And they seem much less stressed.
Thanks for your input, you saved the lives of many fish!

Hi George,

Thanks so much for the update. I’m happy to hear that this one was an easy fix that didn’t require taking things out of your tank, piece by piece, because the airstones probably would have been one of the last things to go. All the best with your fish keeping endeavours!

Hi Ian
This is Iftekhar from Toronto.
I took a project for growing aquarium fish at indoor. My target is buy small fish from whole seller, grow it up , and then sell to local Aquarium store. This is a small scale commercial venture .

I have a 70 Gal. tank with Fluval FX4. But, I made a additional biofilter which is a mini scale of real big biofilter ( using EPS microbead as biomedia which has a large surface area to colonize large number of nitrifying bacteria).
From tank, water first come to fluval FX4, then goes to my mini biofilter, then back to the fish tank.

2 weeks back, I bought 180 goldfish ( each 7 gm.) . It was a new tank .After 4 days .. fish started dying .. now after two weeks, only 40 survived . I know it is due to ammonia spike. I am doing water changing 25% everyday . I use “prime” for de-chlonition of water. Also added bacteria.

Now ammonia is 1.5ppm , nitrite is 1ppm . I think some nitrifying bacteria already developed in my filtration system. But still 2/3 fishes are dying everyday. I am a bit frustrated..

I am using a fine polishing pad in my FX4. My question is, is this fine filter pad can block nitrifying bacteria flow to my biofilter. Is this the reason for getting delayed for developing bacterial colony in my biofilter? Is it possible to block the flow of nitrifying bacteria by the fine polishing pad in FX4?

Kindly let me know.


Hi Iftekhar,

Unfortunately, no matter how good your biofilter is, 180 goldfish is way too many for a 70 gallon. Even once your tank has cycled, there will be deaths.

Cycles are slow and it’s not the fine polishing filter pad that is causing the cycle to go slow. It’s just how the cycle works. I have never seen a fish-in cycle done with 180 fish before. Unfortunately, I cannot advise you on how to make a 70 gallon safe for 180 goldfish. It’s impossible and there will always be more deaths.

Hi Ian
Thank you so much for your comments and advice. So what I understand from your comments, that, even fine polishing filter pad does not have any effect on colonize and growing up of nitrifying bacterias in bio-medias inside the bio-filter.

I am sharing my another recent experience in last October. I stocked 18 Tilapia fishes in 150 liter water( 40 gallon) . Average fish size was 32gm each and total 18 fish weight was 575 gm at the time of stocking. 2 fishes died during grow out time, All other 16 fishes survived well.
After 50 days , I harvested all 16 fishes and Total weight was 1760gm ( 1.76kg) .
The min. size of the fish was 75gm, and 8 fishes were above 200gm. So they gained 1186 gms in 50 days. We fed them only 950 gm. in 50 days growing period. FCR is even less than 1. Feed was 30% protein with 5% fat which is a regular tilapia feed.

The same biofilter ( which is now using for my goldfish) was used to grow up those 16 Tilapia. It took about 28 days to get the ammonia level at zero. I did not use any good mechanical filter for the Tilapia. Only one home made sponge/felt filter was used for removal of solid waste along with my bio-filter. Regular aeration in tank was there.

However, After harvesting, I gave all 16 good and healthy Tilapia fishes to one of my friend.

I did the experiment just to see the efficiency of my bio-filter. Based on that experiment , I started the 2nd experiment with 180 small gold fish. What I understand, Ammonia tolerance is not same for Goldfish as I have seen in Tilapia.

As of today, after 18 days of stocking, 35 goldfish surviving , Ammonia is about 1 ppm and nitrite is 2 , nitrate is 20-30 . Means that bacteria started working. But those fishes already stressed by high ammonia ( like 8 at the beginning) , they will not survive anyway. I have decided now to wait for complete the cycle and then stock again with very cheap feeder goldfish. Lets see.

Thanks Again.


Beneficial bacteria will certainly live in your polishing pad, but your polishing pad will soon clog and need to be disposed of. Ceramic rings or other biomedia are more suitable. But in regards to the polishing pad actually affecting the speed of the cycle? No it won’t have any effect.

18 Tilipia to 40 gallons is a much better ratio, but even then, as they grow, it’s likely you would have seen more deaths if you continued.

It’s a completely different story for your goldfish. You put way too many in the tank. In a 60 gallon tank, three goldfish can be over crowding – they have a high bioload. You adding that many goldfish was a death sentence, there was never a chance that they would survive unfortunately.

Even once your new cycle completes, if you add that many fish, they will very likely die again.

Regardless, I wish you the best of luck in your experiment.

Hi Ian

Thanks for your wish.

Infact, We are in a business of integrating and supplying complete Indoor RAS aquaculture systems in Asia. We have supplied more than 10 projects in India . Each project’s water volume is 180,000 liter ( 47,000 gal.) . Our systems are using for indoor commercial culture of Tilapia, Catfish and Carp. The system has 30,000 liter tank x 6 tanks.
The average stocking density is about 40-45kg of fish( at the time of harvest) per cubic meter ( 1000 liter) of water. Our system includes 80 micron drum filter and and big bio-filter with CO2 stripping system. The bio-filter can handle max. 55 kg of feed per day( it can remove the 95% ammonia generated from 55kg of dry feed everyday). This system replace 10% of its total water everyday with fresh water. This water replenishing system keeps Nitrate value in safe range.

Recently one of our customer in India started growing ornamental fish in his RAS system in commercial basis. He stocks 3gm size 6 types of baby gold fish 20,000 pcs. in a 30,000 liter tank ( 2 fishes for every 3 liter of water) . Grow them up to 12 -15 gm in 65 days and sell it to market. Mortality rate is like 5%. This RAS project runs completely bio-secured environment. This is the 1st. indoor project in India that is growing ornamental fish commercially. They already done 2 complete batch.

The biofilter I made for my Tilapia and Goldfish project, is actually a true small scale model of the big Bio-filter. I am encouraged by the recent success of the Indian project. My thought was to start a small scale indoor system in Canada for growing up and sell Aquarium fish with maintaining water parameters perfectly. So far this a R&D project. It could be a small earning business if it is successful.
Thanks again for your wish. W’ll keep you update

That’s absolutely fascinating Iftekhar.

I think it’s going to be difficult to compare small end-tanks where the focus is on giving a quality of life to a commercial fish cultivating project where the goal is to keep cost and death to a minimum.

I’d be very curious to know the life expectancy of a goldfish grown in such an environment as your customer. Mortality rate is not necessarily a good indicator of health.

I would love to kept up to date on this project. Aquaculture is fascinating, especially if you can get mortality down to 5% or less.

All the best, Iftekhar, I wish you much success in your project.

Dear sir I am from Rajiv Gandhi University, Arunachal Pradesh, India and I am working on some of the hill stream fish in captivity, so as I am new to it the fish are acclimatize ,and maintained but as the winter season came the fish started dieing and my observation on special fish Botia rostrata found to be dying out due to diseases so how to maintain them.

Hi Tinku,

As stated in this article, the most common cause of diseases is stress. Unfortunately, you have not given me nearly enough information here to pinpoint what could be wrong. Do you have a heater to combat the temperature changes within the tank?

Hi Ian

I am having trouble with my betta. His nose is on the bottom of the 10 gallon aquarium and his body is free floating vertically above this. It is a new aquarium to me, but i bought it 2nd hand and on the trip home, had to replace about 3/4 of the water because of the weight of the tank to carry. i replaced the water once home but replacing probably 2 gallons at a time, and by putting in conditioner. all 5 fish were fine, it has been a week now, but 3 days after i got the aquarium the betta was laying on the bottom of the tank, and he has been nose down for 3 days now. i have put in stress coat and water clarifier and i did not change the filter. i believe he will die very soon as this is day six since he has been unwell. any suggestions?

Hi Bonnie

First step is always to grab an aquarium test kit like this one – record your results. This will help narrow down the problem significantly. Otherwise it’s just guessing.

Hey. I’m having a similar problem with one of my platies and I have a test kit and everything looks fine. There’s is an algea bloom that showed up yesterday that I don’t know what to do about either.

Hi there! I just bought a betta fish and I’ve noticed that he won’t eat and is extremely sluggish, just mainly floating around his tank. I looked up what could be wrong and it seems like swim blatters disease. Do you think thats true and do you have any suggestions on how to save him?

Hi Jessica,

Did you cycle your tank? If not, it’s likely a build up of ammonia and nitrite. Read this a lot of beginners miss this step because the pet store doesn’t tell them about it. You’ll need an aquarium test kit to determine what has gone wrong. I use this one.

Sir tomorrow i bought a 6 fish tank then the seller aad one goldfish from another tank in the morning when I got up I saw that a goldfish is being died then I paased away her from the tank after that at this time i see that two fishes is olready died what can i do sir for them they are dying one by one i don’t have any idea for their survival this is my first experience i am so sad sir pliz help me i want to survive my left four fishes

Hi my name is Kendall and I brought a fish betta fish he died after 2 months and then a friend of mine brought two more fish not bettas but Black Moor goldfish and they brought two of them for me that was a week and a half ago this morning I found one of them dead what I’m doing wrong. I’ll try to change 20% of the water earlier last week because it was so cloudy and dirty and I felt bad about it because it was still looked like bad dirty water. so what I did Monday I just can pick the change to water and I don’t understand why one just died and not the other I am a bad fish keeper. I love the little things but I can understand why they keep dying on me

Hi Kendall,

Did you cycle your tank? Many problems can be traced back to water parameters. Please refer to Section 1 in the article.

Hi so I brought a tank a led 10 and it have been cycling for 4 days now I went to go buy a fish it died so I went back to place got water check out and they said water was good got another fish but seem like that fish going to died the first fish stayed at top of aquarium and now the second one is doing the same but at first when I put fish in he was moving and also ate I don’t know what to do from here because I did everything right if they told me water was good

Hi Valarie,

4 days generally isn’t long enough to cycle your tank. You should be testing the water quality yourself with a test kit, to confirm what they are saying is correct. Check out my fishless cycle guide for more information.

Dear Ian,

I have a fish tank of 240 gallons (community tank consist of Platies, Mollies & Angel fish) yesterday everything was fine but at night I noticed few of platies and mollies came to top corner side I thought it’s normal and early morning all of them died (50+) only two angels survived. What can be the reason for it and earlier few days they looks all fine even some platies give birth too. I am very stressed and sad now. Please explain what can be the reason for it ?


Hi Ali,

Have you tested your water parameters with a test kit? If you have, and everything looks good, then it’s possible a lack of oxygen. Fish often swim to the surface (excluding labyrinth fish, like betta) when they are struggling to breath.

How would I know if I have enough O2 in my water? It’s a 75 gallon tank with 8 fish in it. Fresh water. The only aeration is coming from the over the back filter.

Hi Ken,

Unless your fish are swimming to the top of your tank, gasping for air or your tank is overstocked, O2 is the least likely cause of death in a tank – if you are providing surface agitation with an appropriately sized filter, this is typically enough. If you are extra concerned, there are O2 tests available, but in most instances, I wouldn’t even bother.

Hi Christian,

I don’t really understand your question. If you are talking about cleaning with chlorinated water, then near instantly.

What if none of the forementioned apply? We have a 10 gal aquarium. We had 8 koi/goldfish, not positive which. We would lose one a month on average and about to lose the last one. I changed the water (about 20%) and cleaned the tank once a week, and would add one gal if the water got low between. I made sure the new water sat for 24 hrs to become the same temp as the water in the tank. I fed them once a day, i use prime as a declorinator. The fish would start almost laying in the bottom of the tank, and kinda bend in the middle upon death.
We cycled the tank before adding fish, started with 4, and added a couple more here and there up to 10.

Hi Edward,

Reason 3: Overcrowding applies here. Your example is the very definition.

If you have 8 goldfish in a 10 gallon your tank is overstocked. If you have 8 koi in 10 gallon, then even more so. I’m not surprised you are losing fish. You can’t keep adding fish and hope for the best, the “one inch per gallon” rule is a myth. Use a site like Aqavisor can help to an extent with determining stocking capacity, but the best way is to speak to someone at an independent fish store or fish club, who has experience with the fish you plan to keep.

Help! I have a new aquarium , marineland 38 gallon bowfront with penquin 200 biowheel filter. I set up the tank, conditioned the water, added bacteria, let the aquarium “do it’s thing’ ( I thought) and purchased my first 4 tiny fish, 3 skirtfin tetras and a dwarf gourami. Within 4 hours the tetra were dead. The next day the water got quite cloudy, so I figured I’d pushed things too quickly and the cycling hadn’t occurred, so I waited til the water cleared (it’s Crystal now) and tested the water before buying new fish. Again , within 6 hours 3 healthy, active skirt tails are dead. It occurred to me as I watched them that they were struggling with the current created by the filter. I’ve noticed that the gourami , who has survived through it all, tends to hang out on the far side of the tank behind a large plant. I though it was a territory he’d established, but maybe a respite is more likely? Could the flow be what is stressing these ti fish? If so, what to do about that? My water parameters are 0’s for ammonia, nitrite and nitrate, as well as 0 for chlorine and chloramine, pH 7.2, alkalinity 80, hardness 75, temperature 78.

Hi Vicki

If you are measuring zero for nitrate, then it’s a sign that your tank isn’t cycled. Nitrate is continuously going up in a cycled tank and a zero is a sign that something is amiss.

As for the current, fish like betta and gourami live in slow moving water and can indeed be stressed by faster moving currents which can lead to death. However given the speed at which your fish died, I am doubtful this is the cause.

Looking for some advice, as I’ve had 3 mollies die in recent weeks, 2 of which are a bit of a mystery, and the third died of dropsy.

I started my tank (120l/30gal) at the end of January and began a fishless cycle using ammonia. I then tested the water every day until the nitrite and nitrate began to register. I allowed the ammonia and nitrite to return to zero, then added some fish. There was an initial spike from adding the fish but this quickly settled.

I initially added 6 small mollies to the tank, which were all living peacefully. These were allowed to settle in the tank for 2 weeks, then I added a Betta and 3 panda corys.

Everything seemed to be going well, until I came to check on the tank one morning and one of the mollies had died suddenly. Within a few days, another developed dropsy and also died suddenly.

I performed a water change (40l) with mostly RO water, as the water we have is quite hard (off the scale on an api GH & KH test). I always use a dechlorinator with each water change. It was also recommended that I use some aquarium salt to help lower the hardness and add some benefit for the fish. I have been doing a water change of the same size every 2 weeks.

I performed a full water test yesterday, both ammonia and nitrite are zero, and the nitrate is around 20ppm, KH and GH of around 200ppm, with a ph of around 8.2.

From what I can tell (despite a slightly higher pH than ideal) the water quality is good.

All other tank members look great, are acting normally, and feeding well (I feed a pinch of fluval bug bites food once a day, and fed them some frozen brine shrimp instead more recently) and there is never any leftovers.

I monitor my parameters closely, so have a decent data trend with nothing out of the usual showing from my figures, and I’m beginning to worry that something has gone awry that I have missed.

I’d appreciate any advice you can give!

Hi Sam,

Unfortunately, it’s really hard for me to diagnose fish deaths, and it’s outside of the scope of this article. I would suggest joining an aquarium forum like fishlore, local aquarium club or discussing with your local fish store, who may be able to shed light on the matter.

Here is the thing, it might not even be anything you have done. If all three mollies were purchased at the same time, it IS possible they were already stressed or sick due to handling or conditions prior to being purchased. Often, fish don’t show emotions or obvious clues that they are sick or stressed, and it unfortunately might have finally caught up to them a month or so later. This isn’t usual, despite you giving the best life possible. I mean, sure your pH is a little high, but if you acclimated your fish and it has remained constant rather then fluctuating, then I don’t see it being of huge concern.

If you are confident you haven’t done anything to cause the deaths, and your other fish are fine, then I would try buying another three from a different source.

Please can anyone help I have 4 goldfish and fan tails in a 90 litre tank with a pump, yesterday I did a full tank clean (which I now no I shouldn’t have done) 🙁
One of my fish died within 6 hours of the tank clean now the other 3 are acting strange there st the top of the tank gasping they have done this all day, is there anything I can do to prevent these dying x

Hi Jaide,

When you say full clean, does this include the filter media? If so, you might need to re-cycle your tank. Grab a test kit to make sure.

Help please, I just got a 20 gal tank tetra tank with a filter and water temperature. I set up the water stone for aeration. And for only 5 days; 2 fish died(goldfish) one instantly. The second unknown. Later on, I noticed the tank water was getting cloudy and I did a hefty(50%) water change, cleaned the gravel, cleaned the tank walls, and added 2 ph tablets in case there were ph issues. They’ve now all day(11 fishes total; some goldfish, some scavengers and some other cool looking fishes; sorry I don’t really know the names). They never fought/bullied one another and I noticed they gasped for air a lot. But I’m sure that wasn’t the only issue. Please help if you can so I don’t have to accidentally let this process happen again. Incredibly devastated.

Hi Dale,

Likely stocking issues and that you didn’t cycle your aquarium. I’d read up on these two issues, as you are going to lose a lot more fish if you don’t understand these processes.

You only have a 20 gallon tank. This is only enough for ONE goldfish. TWO if you are experienced and willing to do extra water changes. Your fish are dying because your tank is overstocked and probably dirty. Also, goldfish really don’t do well with other species. So I am not sure what you bought, but no doubt it isn’t compatible with goldfish. The air gasping was likely due to low oxygen levels paired with burned gills from high ammonia. You should do some research and then start over. The fish that died instantly died from some sort of shock or perhaps chlorine poisoning. The fact that you added all those fish to this tank but are still asking this suggests that you didn’t even read the article… Smh

Tough love- I love the truth. With access to the internet, there really isn’t an excuse for all this overstocking. People will you please study up on everything before you purchase anything having to do with living breathing creatures? Thank you

My aunt has an aquarium and has fish dying. She recently had radiation for cancer and had to stick her hand in the aquarium. Is this what is killing her fish?

Hi Danielle,

I’d be doubtful that this is the reason her fish are dying. It’s far more likely to be something else on this list.

I have a 25ltr tank and my new filter can go in a tank up to 300ltr is this to big for my goldfish. My 5 yr old goldfish died 6 days after putting new filter in and since I have bought 3 new ones and all died within a week.

Hi Dawn,

If you replaced the filter, you likely crashed the cycle in your tank. If this is the case, you should cycle your aquarium before attempting to add any more fish. Also, a 6 gallon tank is too small for goldfish, so that likely wouldn’t help either.

I have a 78 gallon tank with a mix of koi, goldfish and one minnow. The goldfish and the koi seem to get along nicely with the minnow…so you can mix minnows with such large koi. My largest koi Sunflower, is about 7.5 inches long! While Daisy is about 6.5 inches long! And also, my favorite goldfish is Finch! Ive had had him for about 3 and a half years now and he just lost a scale, i can tell because there is a white spot where the scale had been.

I have a 10 gallon tank with 2 pepper Cory catfish and 4 red wagtail platies. My platies are just hanging out at the bottom of the tank hiding behind decorations and within the first few hours of having them my Corys started hanging out at the top side up like they were dead but they swim around and act active occasionally.

Any advice?

Hi Chey,

I highly recommend researching how many fish you can fit in a 10 gallon (it’s less than you think.) Your tank is too small for the amount and type of fish you have and is a recipe for disaster.

Nice love it. I love all your tips will be greate for the new Betta fish I’m getting. Thanks so you much you really helped me out.

i have had a 20 gallon tank for about a year and a half. We haven’t had any fish die in about 9 months, Just last week they started dying again. I have lost one every day for about a week. Also algae is growing rapidly in the tank like i have never seen before. i have done partial water changes, put stress coat and amno lock in the tank every time i add water. I just took all the plants out, rinsed them and vacuumed the bottom on saturday. Today is monday and it looks worse than it did on saturday. The ammonia is the only thing that is testing high but i have had this problem
before without fish lose and the rapid algae growth. Any advice would be helpful.

Hi Shannon,

First point of order is fixing your ammonia problem. Two likely probable causes: Either your tank is overstocked or it isn’t cycled. In a properly stocked, maintained and cycled tank, ammonia should always be zero. Ammonia poisoning can happen over time, it isn’t instant, so if your fish were already stressed from ammonia poisoning, it’s possible that in there weakened state that something else, or the ammonia, finished them off.

i had 10 gal tank, I had 5 goldfish very healthy for over a year, decided to get the octagon 30 gal tank. Within 3 months my fishes started dying. Changed the water started over again haven’t had any luck all my goldfish keep dying within 2 weeks, got water tested at petco, Water is perfect but they keep dying i’ve killed over 20 fish, HELP!

Hi Barbara,

Firstly, it amazes me you were able to keep 5 gold fish in a 10 gallon tank for so long. That tank is far too small and even though they were not dying, they were likely suffering. I would suggest reading up on how many gold fish you can keep in a tank.

Did you cycle your new tank before adding your goldfish? Also, I would recommend buying an API master test kit and performing the tests yourself. A $20 kit has all the tests you need.

It’s also possible that while the new tank is larger, the surface water area is smaller, which means less oxygen for the fish, although I cannot comment on this without seeing your tanks.

I am new to the fish world. My daughter talked me into a Betta 3 months ago. He has been great in his bowl (the ones with the top for a small child) until recently she wanted a few more fish. I upgraded to a 10 gallon tank and added a few neon tetras. they quickly died 🙁 NOW my Betta has what looks like his nose/mouth is “peeling”. I removed him this morning from the tank and back into his own bowl. he was laying at the bottom of the bowl and would only come up for air and would float back down. That was when I left this morning. I have now read about “cycling” your tank prior to putting the fish in. no one at the pet store told me that! is there any chance he will live!? Thank you!!

Hi Courtney,

Unfortunately most big chain pet stores don’t discuss cycling. They would sell less products if people realized they had to wait before they can add fish to their tank. This is why I recommend local fish stores – they usually stress the importance of cycling as they are in the hobby too.

It’s hard to say just what the disease is but it was likely brought on by the stress of an uncycled tank. It’s also possible that this stress was left over from when he was added to his first bowl, sometimes it’s a slow process.

This doesn’t mean your fish will die. clean water is the first step, so grab an aquarium test kit and confirm your ammonia, nitrite and nitrate. If these are elevated, you’ll need to bring them down. From here it’s medication, depending on the disease. It could possibly be columnaris but your description matches a range of diseases. Unfortunately, I cannot say for sure whether your betta will make it or not.

Hi, I have a 10 gallon tank, cycled it for 2 weeks. Got water tested. Got 2 new fish and they only lived for two weeks. Cleaned the tank again, cycled it again for two weeks. Got two new fish, one died yesterday and now the other one is staying towards the top of the tank. The water tested fine again. Pleas help. I have a tank before and never had this problem. My kids are devastated about not keeping fish alive.


I have 5 neon tetras in a 10 gallon tank. We’ve had them for about 3 1/2 years. There is one fish that has always been smaller than the rest. He’s been having a very hard time lately. I’ve noticed for the past month or so he’s been hiding inside of the Squidward or Spongebob house decorations we have. There is one fish that’s kind of a bully – so I figured he was hiding from him, as he comes out when it’s time to eat (they can tell by me opening and closing the drawer underneath the tank stand). This morning I noticed he was having a very hard time swimming. His fins also look kind of mangled, and he looks rather pale. He is just sitting on the bottom of the tank gasping and having a rough time this afternoon.
We had some problems with fin rot with all of our fish around a year ago – but I treated the tank with the tetra lifeguard formula and all was well.
The other fish seem to be doing well as usual.
What is going on and what can I do?
I put some tetra lifeguard in there this morning – thinking he might be having a problem with a bacteria or fungus of some sort.

Hi Danielle,

Unfortunately, I am not qualified to diagnose your fish. It could be a disease, bullying or just the end of his natural life (not all fish live to a ripe old age, particularly those born with defects)

Before 4 days i bought a fish bowl which is capable of 1 gallon water. Along with the bowl i bought 2 goldfishes, one is 5cm long and another is 7cm long. The bigger one eats all the food i drop on water and not allowing the smaller one to get the food. If the smaller one tries to catchup the food, then the bigger-one moves faster and bombards with the smaller one. Also it(biggier one) chases the smaller one if it tries to catch the food. The smaller one fails and it is not eatting.. It only eats two or three pieces of small-mustard sized fish food. But the bigger eats countless pieces of fish food.
kindly give me solution for this problem..pls

Also i found that after every 12hrs of time both the fishes are comming to the water-air surface line and gasping air. when i drop food, it doesn’t eat and continues gasping of air. I found that few wastes are flying inside the water. After i change water, they will swim inside and not comming towards the air-water line and also it eats food. For every 2 days, am changing water.

pls, help me understand about my two new friends.

Hi Soundarya,

All your problems stem from a 1 gallon tank being way too small for your goldfish. See point 3 in the above article. In an appropriate sized tank, there would be room for both to get fed and enough oxygen in the water.

Ok sir, i will provide new home for them soon. As a new person to this fishworld, i really feel like am having a right person to guide me. thank you sir. I request you to continue your guidance.

hi sir, am soundarya.
I bought a new fish tank. I thought that my two goldfish fishes would be happy. But my aunt presented me another two new fishes named “white koi carp fish”. They appear little rude. My bigger goldfish is as-usual happily enjoying the new tank along with two little white koi fishes. But my smaller golden fish as-usual being alone in a side of tank. Two koi fishes are sometimes hitting and chewing (i thing they may give little bites) the small goldfish.
what can i do now?.. If this continues, my little goldfish may be stressed-out heavier. Am afraid now. Do white koi fishes eat my little goldfish?..

Can i again put my little goldfish in another 1 gallon bowl? if i do so, the little goldfish feel happy?

or my little goldfish will be comfortable if days pass?

Hi Soundarya,

Unfortunately, koi are best kept in ponds, they grow up to be huge. Once again you have the same problem where you have too many fish for the same size tanks. Goldfish cannot be kept in 1 gallon bowls, you will have the same problems that you had that we discussed before.

Hi so I have a 10 gallon tank with one goldfish. I’ve had him for about a year and he was a great fish. I changed water, cleaned the gravel, and changed the filter appropriately and did water tests weekly. I fed him a couple of flake food everyday(making sure for it to sink so he doesn’t gulp air) and occasionally gave him pellets with more veggies so his diet wasn’t boring. He was beautiful and very happy. He loved to chase my finger around and stare at me lol. A few days ago he started having issues swimming and floated upside down. I looked it up and found swim bladder disorder. It said to have him fast because the main cause was from gasping air while eating food(even though I made sure it sank for this very reason) However I did not feel comfortable raising the temp or adding salt. I tried helping him swim right side up but it was useless. He just died and I’m very sad. He looked perfectly healthy. But his fins were a little torn from rubbing against the gravel so much. This has never happened to me and I don’t know what I did wrong 🙁

Hi Elizabeth,

While 10 gallons is too small for a gold fish, it sounds like you looked after him to the best of your ability. This is perhaps the most frustrating thing about fish keeping, even though we do everything right, it might just be the fishes time to go. Birth defects and even diseases beyond our control can take hold and there is little that can be done. I know it’s never easy to lose a beloved pet, I really feel for you here. If your water parameters were otherwise normal and you did everything you say, then you gave your pet the best life possible within your means.

Hi. I’m really worried about my beautiful fighter fish. He has become really sad and either sits at the bottom of the tank not moving for hours or rests high up against the filter! He is hardly moving and I have had one die this way before. It’s so sad. I have done a 50% water change with the declorinator. Added 2 little bacteria balls to the filter. Used some melafix as it’s fins look a bit rough! The other fish are fine but I don’t know what else to do and i’m Worried that I get up tomorrow it’ll be dead! Help!

Hi Rachel,

Unfortunately, I can’t diagnose your fish over the internet 🙁 have you done the basics listed in this article like use an aquarium test kit to test your water?

I’ve had my 45 gallon tank for a little over three years. I’ve maintained my tank to the best of my ability. About 2 weeks ago my fish began to die off 1 by 1. Out of 19 fish I now have 6. I had my water proffesionally tested and it passed with flying colors. Nothing has changed in my maintenance regimen. Just out of nowhere BOOM dead fish and cloudy water. HELP ME PLEASE

We’ve had our tank for a year now, been through so many fish bc they all die within a matter of months, specifically the angel fish. The nitrates/nitrites reading ok, 55 gallon tank with suitable filter and enough air for all 8 fish. The latest angel developed dropsy I believe. We dont know what to do. The water recently around a couple months ago stopped turning green all the time and we thought that would help with all the fatalities but no luck. Any suggestions are welcome!

Hi Jessica,

Unfortunately, there isn’t enough info here to pinpoint anything that could cause the death of your fish. My suggestion would be to work your way through this list and cross them off one at a time.

As for your readings:

Ammonia should = 0
Nitrite should = 0
Nitrate should forever be increasing (your weekly water changes get this back down under 10, ideally in a non-planted tank)

You should be using a liquid test kit to test these (I recommend the API master test kit) since test strips are both inaccurate and more expensive over time.

I have noticed a lot of people buy neons as a first fish…I have found that neons need a well cycled tank of 6 months or more..they also get really stressed when you buy them..I no longer look on them as first fish..I have found black neons a better fish even if they are larger and not so colourful..

My sister used to have a frog and a beta fish they lived in the same tank and had a happy life, the frog eventually died and the beta fish not soon after. Recently we got her new fish, 3 painted glass fish. After only about only 1 week of owning them they started dropping dead one after another each day, and know they are all dead. We really are unsure of what is happening, the water was clean, they were feed appropriately, but they just died.
I don’t know if this is enough info, anything helps,
thanks for reading,
~ A concerned and curios fish owner.


When you say your water was “clean” do you test it? In fishkeeping, we cannot tell how clean water is without an aquarium test kit (I recommend the API master test kit) If you didn’t cycle your tank before you added your fish, then this is the most likely cause of them dying.

2.5 gl tank, small filtration on side of tank. 2 neon tetras and 1 betta fish. Tetras got sucked up I to filter while I was at work and came home to two dead tetras. My betta is getting stuck in the current, filter too strong or wrong side

Hi Kat,

Bettas typically don’t like much or any current at all. You can reduce the water flow by using a piece of sponge on the intake of the filter.

I’ve had a 20 gallon tank for a few months now. I had one rainbow shark fish, four guppies, and two snails along with four plants and some decorations. For a couple of months they were well and going about their business. In the past month or so I added three tetras, the shark fish was having a good time chasing them around the tank. The. I added one guppy that gave birth to two babies and got pregnant again by the male guppies. So I went and bought a Molly to take care of the guppy fry (still not here yet by the way). But I was addicted and couldn’t stop so I bought another snail and two more guppies. They lived harmoniously together for a few weeks, then all of the sudden in the span of three days they all died, except for the baby guppies (for now). I have no idea what’s happened, I get my water tested daily at petco and every time they tell me the water is perfect. I’m now officially depressed and blame myself. I don’t understand what happened, the fish people at petco don’t know and I can’t figure it out.
Please help me understand what I’ve done wrong.

Hi Maryam,

Petco is the last place I would go to for fish advice. You should have an aquarium test kit on hand (I recommend the api master fresh kit), this will give you a good idea of what your water quality is – which is responsible for the vast majority of fish deaths.

However, based on what you have written, do have a suspicion that overstocking is the problem, leading to poor water quality. A test kit will confirm this.

I do have the test kit you speak of, the only thing the test kit showed was a 7.6 ph level. The rest came back normal. I have also bought the ph reducer by imagitarium, and have tried to bring the ph down. I am now left with one guppy baby ( the other one died between the time I wrote this and your response) , one pregnant guppy (which is hovering around the heating element), one other guppy and the Molly. What do I do now? How do I keep them alive? Also how many fish can I have in a 20 gallon tank? I’m desperate, please help me.

Hi Marayam,

When you say normal, what were the readings of each parameter?

Ammonia –

Nitrite –

Nitrate –

A pH of 7.6 is fine.

The amount of fish will entirely be determined by the type of fish. For guppies, the general advice is 3-5 fish per 10 gallons of water.

Good morning,
Ammonia has been around 0ppm, nitrite has always been around 0ppm, and nitrate has been around 0ppm. I’m using API Freshwater Master test kit and on top of that I’m using the JNW Direct aquarium test strips.
Is it possible one of the new fish I got was sick and killed the other ones?

Hi again!

Let’s see if we can’t get to the bottom of this.

If nitrate is always at 0 then that isn’t really normal. Unless you are doing non-stop water changes, or have chemical media that will absorb nitrates, it should continue to rise non-stop in a healthy tank. Part of the reason we do a weekly water change is to lower nitrate levels and a lack of nitrate can suggest an unhealthy tank or a test kit that isn’t working.

Also are you following the instructions 100% to the letter of the API master test kit, if they are not followed perfectly, the results can end up wrong. Also are you checking your test kit results in natural daylight? Indoor lighting can throw off the results.

On the sickness front, it is possible, but for a mass wipe-out like this, there would generally be symptoms of something being amiss.

Good morning,
First, I would like to thank you for all your help and guidance and let you know it’s appreciated greatly.
Also, you are correct. I do a lot of water changes (about 5 gallons every few days) because I’m terrified of leftover food in the tank. Specially when the water b comes murky. I do follow the instructions to a T, it’s usually me doing the testing and my boyfriend reading the instructions. We do not look at it in the daylight because we usually test the water when we get home from work. I will try and test the water this weekend during the day.
The issue I’ve been seeing is when the food is left behind the ammonia starts to rise so I immediately do a water change. On Tuesday i did a whole tank water change and washed the gravel with a lot of hot water and wiped the tank clean with a wet paper towel, no dish soap or any cleaning agent was used. My water hardness seems to be a bit high, and my boyfriend has bought a water hardness reducer thing ( not sure what it is, it looks like a fabric bag with yellow stones in it). Not sure if that’s working or not, but he puts it in with the filter for two days and then takes it out and soaks in in some water and salt solution ( I think). Aside from my frantic water changes I’m not doing much of anything else. Although my boyfriend thinks I over feed the fish, i also have two cats and i evidently over feed them too :’)
And again, thank you so much for your help.

I think we may be getting closer to the issue, at least based on what you have said. A tank shouldn’t become “murky” before the weekly water change and the ammonia rising can be a sign of one of the following:

You don’t have enough biofiltration (often happens when you use “disposable filter cartridges”)

You have too many fish (that poop and pee adds up)

You are overfeeding waaaay to much, which you have partially identified. On the overfeeding part, you would need to research how much each individual fish needs, it’s often less than you think. For example, single betta only needs 3-5 of those really tiny micro pellets.

In a properly stocked, you shouldn’t need to change the water out/clean it any more than once a week.

hi so i got a fish from my teacher (a goldfish) my sister got jealous so she and my dad got fish. the next day my sister fish was dead I didn’t think much of it cause her fish was small so we got her a new fish. The next day when I came from school my fish and my sisters fish were both dead. All the fish have died except my dad’s .

Hi Mirna,

Very likely point 1 in the above article. New fish tanks need to be cycled. There are 2 links in the step 1 there that your family should read before buying any more fish.

Great article. I’m fighting a mystery fish/shrimp killer in my 10 gallon tank. This tank has been established for years with dwarf puffers and a Siamese algae eater. IT is moderately planted. I fed them tubifex worms from my local fish store. I decided to switch directions and traded in those fish to the store to start something new. Since then, almost everything I have put in that tank has died. Molly’s, Platties, pleco, all kinds of shrimp and a few others. Currently I have 5 neon tetras as a test. They don’t look happy. Even the my crypts are starting to have problems. The only fish that seems fine and will eat food is a small rainbow fish. The tetras looked stressed out as they hide on the bottom gulping all the time. Tank water parameters are normal, I have an air stone, plenty of filtration (oversized filter) and a good heater.
Ammonia – 0
Nitrate – 0
Nitrate – 5 to 10
Phosphate – 0
PH 7.4
GH – 125
KH – 30
Light may be a bit high for the tetras but is for the plants. I feed the plant with root tabs every 4 months. I do water changes weekly, though I may be doing more than 20%. I light vacuum the substrate, which is active-flora. Local fish store had me check for electrical leaks with a multi-meter and although there is a small bit of voltage, it is the same as my 16 gallon, which is vigorous and happy. Latest theory from LFS is anaerobic pockets in the substrate (2-3 inches deep). I did deep vacuum the substrate in a corner of the tank and found areas of black and white cloudy film which may support that theory. I don’t seem to have any bubbles or Sulphur smell. With no clean up crew, I am developing a brown “algae” on my leaves which rubs off easily. No phosphates in the tank so don’t believe it’s diatoms. There are still tubifex worms living in the substrate since nothing in the tank will eat them right now. I’m close to throwing in the towel and restarting the tank. I would appreciate any insight you may have from my description.

Hi David,

It’s incredibly difficult to diagnose online. These are the types of questions that are best asked on forums like fishlore or – where someone else might recognize the symptoms as having happened to them and weigh in with a solution.

However, if it’s the anaerobic pockets in your tank, then stirring the substrate with a chopstick should release them and you’ll be hit with an awful sulpher smell.

I think I’ve said this before…

but goldfish can grow up to 15 in, sometimes larger, not 10, and can live to 30 years, not to 10 years. 10 years is often the minimum for goldfish.

Hi R,

It’s just an example used in the article to demonstrate the difference in fish life span to beginners. I said “live for 10” not “to 10”. Thanks for weighing in.

Hi, I read this blog when I had 6 new Ember Tetras yesterday for my 10 gallon, and acclimated them (after cycling the tank for 5 weeks) and I was sure I had done it wrong! I’ve read your information about cycling and everything else, about Kh and pH and lots of information that has really helped me. So I tried to find out what I did wrong with acclimating them from this post. Couldn’t find anything, went to my local good aquarium store, they said ‘check your filter, even if you don’t think it’s possible’. I had a shrimp filter because embers are so small! All the inflows and outflows of water were covered with grid and foam! What it turned out to be was that the embers were swimming in a -3 mm- hole in the back that was mentioned for a suction cup that had fallen out!! I lost 4 of my 6 new embers! I am an absolute beginner though I’ve read everything and I was sure I had done something wrong. So, if your fish ‘disappear’ and you can’t find any bodies, look in the filter even if you think it’s not possible!

Hi Amaranth,

I’m terribly sorry this happened to you. Thanks so much for sharing though, I hope it helps someone else who has fish “magically” go missing.

Hey Fish Lab,

I’m a beginner fish owner and quite by accident. Last week I had 5 common goldfish and now . . . I have one. Truly their tale must be the worst for fish- won as a prize in a festival game. I pittied the poor fellows and decided to try to give them a better life. I have done nonstop research during my spare time trying to save these poor guys and just when I think I am on the road to success, another one floats belly up or in the filter stuck (dead then stuck to filter, not filter killed them).

Spontaneous as it was, I had no choice but to buy a tank same day I aquired the 5 fish and attempt to do fish in tank set up the next day (I wanted to give at least one day for the tank to run before putting them in). One died in the festival water overnight- not unexpectedly, it was filthy.

Unfortunatly I only had the budget for a 10 gallon tank which I am aware is not quite sufficent for goldfish who thrive more on 10 gallons per fish, but it was the best I could do. So, after night one I acclamated the 4 fish left and hoped for the best as I transfered them in. A week past and they seemed to actually be doing really great. On the first day they didn’t eat at all and I was worried the biggest one would die since he never moved out of the corner, but suprisingly he is the last one alive.

Two of them died together. I believe one of them had swim bladder disease or bloating as he floated to the top of the tank and couldn’t swim to the bottom. I tried feeding peas to the fish to help but the one at the top of the tank didn’t eat and the same night I fed peas a different fish which had been doing fine became inactive, layed on the gravel floor, and in the morning both the floating goldfish and the floor goldfish were dead and suctioned to the filter.

That depression hit so hard. Down to two.

The biggest one (2 inches) and a much smaller common goldfish (1.25 inches). They lived in harmony for about half a week longer (even got a bubbler they enjoyed swimming in) and all of a sudden the smaller one just gave out (he hung out around the filter staring in the corner night before death, morning of swam about seemingly fine, then a few hours later just flopped). The bigger one did chase the smaller around the tank sometimes, but my research showed that goldfish are social and this behavior is common. The bigger also did become slightly territorial at meal time, but I wasn’t underfeeding and the smaller still ate so I dont know what has gone wrong. Only guess is stress from the chasing???

Anyways, I’m worried that my last goldfish my face the same fate as the others. I feel like I have tried to learn a lot about caring for these fish but I still feel helpless as they die off one by one. (What went wrong with the peas?!).

I am still not sure why they have died. Everything is just speculation. But, I know for sure that I dont want my last one to go belly up.

Do you agree with any of my conclusions about their cause of death? Any tips about keeping the last one alive? Are there any signs of a tank set up being the problem I should check for? What are some signs of under or over feeding? Or maybe this should this all have just been expected from a kiddie pool, festival goldfish game? Really any advice for a beginner who went from 5 living fish to 1 in two weeks would help- signs, errors I may have made, should I invest in a companion for the solo goldfish left, etc. Anything would be awesome.

Thanks for reading

Hi Carey,

I know it didn’t turn out how you planned but I have to congratulate you on doing everything you could to save these fish. I know it wasn’t easy.

Even if these goldfish were in tip-top health (which coming from a side-show game, I doubt it) they were unfortunately doomed from the start. When you first set up a fish tank, it needs to be cycled. To put it simply, this process involves getting bacteria to grow that eat the fishes waste (so they don’t get sick from swimming in their own toilet)

Ideally, you would want to do this without fish. But since you have fish, you’ll need to do a fish-in cycle.

For a fish-in cycle, check out this guide. It should get you up to speed:

However, if this last fish passes, I’d look at getting some more size appropriate fish for the tank and performing a fishless cycle instead.

hello, i have a on going problem. i am not a beginner but not an expert. i have a large 55 gallon take no problems i have a 7+plus year old fish. anyways to the problem tank my daughter ten gallon. first she had only a male and female baby beta, some tetras as well. i watch for any fighting in case i needed to move one beta. anyways we added a two snails and a pelco abd a drawf frog. well the drawf frog died about two days later from what i assume was a fungal infection. well after that and the addtion of the snails eveything kept dieing even after a particle water change. i ended up washing everything out and set it up yet again. and gave her three of my fish from my tank thats been about a week ago. now two have died out of those fish abd just the snails seem to remain doing fine… i get stress can kill. but what else could this be could it be that the snails are the probelm… the first fish were in the tank for well over a month before the frog and snails were added and eith the water changes mostly particle i am very confused and frustrated. i have had that many fish die ever in my large tank unless we talk about the feeders i put in for my jack demsy.

Hi Rebecca,

Your issue was point 1 and point 3 from the above guide. You need to cycle new tanks. And that was way too many fish for a 10 gallon.

I highly recommend reading point 1 and the attached cycling guides as well as buying an aquarium test kit – it will help you identify the most common issues.


I just recently bought 4 glo-fish from petsmart and they all died in my 10-gallon tank. I took a small sample from the fish tank and they tested the water for me. Everything was fine so I thought it might have been the temperature because I had my tank at 72 when they said it should have been at 78-80. So I adjusted the temperature and waited a long time because I got busy. Some of the water evaporated so I refilled it and I think I might have turned the heater’s knob because yesterday I bought 4 new fish and noticed that the temperature was at 73 I panicked because I did not want the fish to die again so I turned the knob. Eventually the temperature went back up to 78 and they all seemed fine. I woke up this morning and 2 had died, the other two were still alive (red and blue). I kept looking at them for some time and the red one started breathing really rapidly and kept getting stuck on the decoration. He died after some time of struggling. The blue one was still swimming normally when I left the house I’m really sad about this. I’ve only ever had a goldfish and a betta before (not at the same time) and neither of them required a heater or a 10 gallon tank. For my betta I had a small bowl and he lived for 3 years. My goldfish lived in a small 1.5 gallon tank with a filter and he lived happily for about 5 years. I don’t know if it’s the stress because all they told me to do was to keep the bag in the fish tank for 20 minutes and then let them go. I don’t want to buy new ones until I know they won’t die. I am planning on changing everything and redoing everything please help.

Hi Valeria,

Did you cycle your tank? See point 1. if you didn’t and read through the links there about cycling your tank.

Also, it sounds like you had hardy fish your description of your previous two fish sounds like it came down to luck. A goldfish won’t “live happily” in 1.5 gallon tanks. It will “survive” – just because a fish is alive doesn’t mean it is happy. GloFish are typically not as hardy and the same approach didn’t work.

This isn’t your fault at all, you didn’t know, like most beginners. However, you now have the opportunity to do things right.


You mentioned in reason #8 that changing the filter/filter media can cause rapid water changes which will lead to a belly-up fish.

Is there a specific way to change the filter so that this doesn’t happen?

Hi Rosie,

Generally speaking, you don’t swap out your filter, you clean it. If you have a filter with disposable filter cartridge, these are a somewhat of a scam – every time you throw out your filter, you are throwing out the good beneficial bacteria attached. At minimum, your filter should have a piece of sponge that is rinsed and can be replaced and ceramic rings that last up to 5 years or more, depending on the brand. When the time comes to replace the ceramic rings, they are swapped out half at a time, so that they beneficial bacteria can move to the new ceramic rings.

If you have a sponge filter, you would run a new, second sponge filter at the same time for about a month before removing the old one.

Sorry, I have another question. Your instructions say once you get down to ammonia levels of 2ppm do a 50% WC. I’m only at .50ppm in the 20G and .25ppm in the 15G, so I’m not sure if I need to do a 50% WC as frequently….how do I handle WC with these lower ammonia readings – for which I’m extremely grateful.

I would also like to comment on what you told Rosie about the ceramic rings. The instructions on the box say change every 3 months and you say they’ll last up to 5 years? I can’t tell you how fast I cancelled an order for those! And nobody ever says anything about changing only half at a time! Which makes perfect sense! It’s all so frustrating when all you’re trying to do is do right by your fish. I realize this is a learning experience as you go along, but that seems like basic tank management 101 and nobody passes along such relevant information – except you. I am so grateful to have stumbled on your website.

Glad I can help!

The good news here is that you can do water changes less frequently. In a normal cycle, there should come a tipping point where your ammonia is decreasing and nitrites are spiking where it may be more frequent, but you are not there yet. At the current stage, 1 water change every 1-2 weeks to replace GH (trace minerals that are essential to fish health) will be fine. Once your tank is cycled, replacing the GH is the other reason you would perform a water change, in addition to getting nitrates down.

Ceramic Rings are a very mixed bag in terms of construction and quality. To keep things simple, I refer to “biomedia” in general as ceramic rings. However, some will only last a year, while others considerably longer. If you go with the “ceramic” style, you’ll know when it’s time to replace them as they will go soft and crumbly. When this happens, the tiny pores on the rings will begin to collapse (which is where the beneficial bacteria live) and each ceramic ring will hold less and less beneficial bacteria.

It’s very difficult to recommend a budget brand since many keep changing their source of ceramic rings and quality varies dramatically. If a brand recommends you swap them out every 3 months, I would suspect they are using an inferior ceramic ring. I would be worried if someone was only getting 3 months out of their ceramic rings. Anecdotally, I would expect most people would see up to 2 years with a ceramic ring like Fluval Biomax. I personally use Sera Siporax, which is sintered glass. It’s easily been 5 years since I added it to one of my tanks. If your budget stretches that far, it will potentially save you money in the long run. Just ignore the claims that it can remove nitrates from your tank. While the science behind it is true, you definitely won’t be able to fit enough in your filter to make a noticeable difference.

Unfortunately, fish don’t come with instructions. They should! Raising them is more like chemistry than keeping a traditional pet. I feel sorry for those parents who get their kid a goldfish as a beginner pet and say “If you can keep this alive for a year, we will get you a puppy” – that kid never had a chance 🙁

Hang in there though, it will get easier. When it switches from “Ahhhh panic, cycle!” to “I can sit back and enjoy my fish” it will be worth it.

Today the ammonia is down from 0.50 to 0.25, so that’s encouraging. I plan to do a WC the day before my surgery next week, and hopefully the water will be ok for about 3 weeks. Fortunately, the tank is close to my front door. If I do 2G and empty the bucket by dragging it to the door, do another 2 G, etc until I reach 10G, I should be ok and able to handle that. As far as the bio media, it’s a reputable company that recommends changing it every 3 months. Aquaclear, or whoever makes the aquaclear filter and supplies that go with it. Not exact a slouch company!

That all sounds positive. I hope it all goes smoothly for you. The advantage of these smaller tanks is that there is less water for you to change. I hope you recover well!

On the biomedia front, aquaclear is one of the brands that has changed their ceramic rings multiple times. Their old ones were superior, although their surrounding bag wasn’t the best – before you had to fill it yourself. Unfortunately, most aquarium manufacturers these days move away from what is best towards what has the most profit margin for them. For instance, Aqueon used to be an amazing brand, making many products in italy. Now their products are some of the worst performing on the market with incredibly poor quality control.

The articles I’m reading here are scaring me. I have a 15G with a divider and a betta on each side. The tank is fairly heavily planted. Substrate has bacteria in it, there is bacteria media in the filter. Plants are showing new growth. My nitrates and nitrites results have been 0 from day 1. pH is steady at 7.5. I “cycled” this tank for a month before adding fish. I do a 2G WC/once a wk. they seem to be doing ok. The 20G has the same water parameters, same filter, substrate. It was without fish for 5 weeks. Then I added 3 dwarf neon blue gouramis. A week later I added a pair of guppies. Yesterday I did a 5G WC, today 1 gourami died and the other 2 don’t look that good. The 2 guppies seem to be doing ok. What I did wrong I realize now, was adding room temperature water to the tank. The tank temperature dropped from a balmy 79 to 73.2 and I believe the gourami went into shock from the sudden drop. Before I do any more WC, I’m getting a cheap heater to heat the new water in the fish-dedicated 5G bucket. The water I use is reverse osmosis because I have well water that’s as hard as a rock, so chlorine isn’t an issue, but I do let the water sit in the bucket for a few days before adding it to the tank. I have at least 2 issues. Why have there not been any spikes in nitrates and nitrites this whole time? Even with fish in both tanks, they still test 0. How many days before I can consider my guppies safe from the sudden drop in temperature? It seems they’re more tolerant of the drop than the gouramis, and maybe a bit hardier. I’m very interested in your thoughts and comments. Thanks for your help. BTW, I’m new at this, but spent months reading, talking to fish store staff (not petco or PetSmart, real fish store people) and watching YT videos from people that do know what they’re talking about, so I felt I was going in the right direction, now I’m not feeling so confident.

Hi Deborah,

If your nitrates are at zero then something is wrong. In a healthy cycled tank. Typically, nitrates continue to rise until you perform a water change to help get the levels back down.

First, What are you using to test your aquarium? For most people, I recommend the API master test kit, it will last for years and has everything most tanks need. Test strips are not accurate. Next, you want to make sure you read the tests in natural daylight, indoor lighting can cause the colors to be off and give an incorrect result.

You also don’t mention your ammonia results. This is just as important as nitrite and nitrate. If you are not measuring this, it’s difficult to say what is happening in the tank.

Otherwise If you have a filter media (such as zeolite) or an additive that removes ammonia or nitrite, then this could be the reason no nitrite or nitrate has been detected. If this is the case, then your tank never got a chance to go through the cycle, and you’ll need to start from step 1 without the ammonia remover.

On the fish front, it’s hard to say. If it’s a one off, and everything else in your tank is otherwise good (which from what you have said, it might not be) then they will likely make a full recovery, but there are a lot of “ifs” here.

You were so right. Those test strips are worthless. I got a master test kit here’s the results: 2 betta 15G – pH 7.4, ammonia .25ppm, nitrite 0ppm, nitrate 5.0 ppm. 20G 2 gouramis, 2 guppies – pH 7.4, ammonia.50 ppm, nitrite 0 ppm, nitrate 5.0 so what’s the best thing to do at this point? Add bacteria in a bottle to both tanks? Or add de-ammonia solution, or both? Everybody in both tanks seem to be doing fine this morning. At least they’re all alive, I’d like to keep it that way…..

Okay, this is all good – we now know what we are working with!

Unfortunately, your tank is uncycled. But the good news is your ammonia levels are low, which is where you want to keep them in order to keep your fish safe.

For how to best move forward, I recommend reading this fish-in cycle guide. It will take you through the steps to keeping your fish safe throught the cycling process.

Also, I recommend testing your tap water before you add it to your tank. 5 ppm of nitrate wouldn’t be expected yet. However, some people do have this amount in their tap water. If your tap water does measure 5 ppm, this becomes the new baseline for nitrates in your tank, instead of zero.

Wow! Thanks for all of that. Fortunately, I have a 500 ml bottle of Prime, which I believe will be enough to get me through this with both tanks. I also have a box of droppers marked in increments of .25 ml I did my first dose today. Not sure how I’m going to handle all these WC. I’m having rotator cuff surgery on my dominant shoulder next week. I’ll be in a sling for 6 weeks. ???? I have reverse osmosis water because of awful tasting well water. That’s what I use for my tanks. I tested the nitrate out of the tap as you suggested and it was 0 ppm. Also, why do you say this process is hard on the fish? If the Prime is holding ammonia hostage, then aren’t the fish safe? Or is it daily doses of Prime that’s dangerous? Thank you again. I don’t know how you keep up with all the inquiries.

Sounds like you are all equipped to tackle the cycle then!

Are you able to call in a neighbor, friend or family while you are recovering? Water changes are the most direct solution if something goes wrong, or if you need to drop the ammonia and nitrite to a point where the seachem prime can once again deal with them.

If you have RO water, you’ll also want to watch your KH. Soft water (which RO water is) can cause pH to bounce around, which is yet another thing that can stress out fish. I have a guide on that too:

If your pH bounces around, I would suggest reading up on this.

The process is hard on fish because in an ideal world, we would be on top of things. But life happens, test kits (while good enough) are not “perfectly accurate” and a wide other variety of factors means that even in the best cycle, fish will be exposed to some degree of poor water quality. It’s unavoidable and sadly many beginners lose fish in this process. However, you have caught this while your ammonia is low, so there is a good chance your fish will make it out the other end!

On the Nitrates reading 5 but tap water zero, be mindful that *some* people experience a different cycle where nitrites forever stay at zero but nitrates increase. While this is uncommon, I have seen it happen enough times to mention it. If this is the case with your two tanks, don’t panic. It’s still a normal cycle, just slightly different and you’ll still reach the end!

Hi! I contacted you a while back about an ammonia problem I’m having. I’ve been doing 25% WC 2x/wk, adding Prime daily but it’s not getting any better. I started adding API BB daily, which API suggested I do until this is resolved. I only have 5 guppies in a 20G tank so it’s not overloaded. It goes from 0.25 to as high as 2ppm and everywhere in between. Nitrites and nitrates are negative, pH is 6.8 and steady. I am very careful about not over feeding. It’s heavily planted and I’m seeing new plant growth. What am I doing wrong or is there something else I should/could be doing. I’ve heard the API ammonia test is not always reliable. Is there a different brand that is more accurate? Thanks for any help you can give.

Hi Deborah,

If you are using the API liquid test kit, then it should be perfectly fine. The API master test kit is the one I recommend most for it’s ease of use, affordability and that it is accurate enough for most people. While it IS possible that you have a faulty test, in my opinion it would be very unlikely.

I couldn’t find your last message, but if your nitrates are at zero, then it sounds like a cycling issue.

Have you read this article:

I generally don’t recommend adding BB boosters since you can cycle a tank without them, but it won’t hurt your tank if you do.

I am experienced – decades of keeping reef tanks, planted tanks, all sorts. Currently have 3 200G+ tanks – full reef, planted discus, african cichlid. A couple years ago I established a 3 gallon fish bowl with no filtration, heater, or even an air stone. I set it up in my kitchen and did weekly water changes from my planted discus tank with zero nitrates. there is no bio filtration in the bowl, yet the five white cloud minnows (that live at room temp) have all survived 2+ years and have bred – there is now a sixth. With this success, I set up another bowl, but at my office – too far for water changes from the established tanks. Instead, I use tap water treated with Prime. Everything else is the same. Yet this fish keep dying. It must be either 1) over dosing Prime or 2) the used fish tank water from the established tanks has bio/bacteria in the water column. What is your expert view on this?

Hi Steve,

There are a LOT of factors that are come into play here, from the right temperature (From aircon causing larger than expected temperature fluctuations) to the pipes in the building (they could be contaminated)

While beneficial bacteria (the ammonia/nitrite oxidizing variety) doesn’t really exist in the water column since it is a clinger instead of free floating, it is very possible that one or or a combination of bacteria from the other tank are keeping things in check.

Thanks for the reply. Following on the assumption that Prime was eliminating oxygen from the water column I added a small air pump with airstone and immediately the fish perked up and have been thriving ever since. They are now eating food as well. Simply put, I think the airstone adds oxygen and water movement that was missing previously and this seems to have changed the bowl for the better.

Hi Steve,

That’s really awesome to hear. I’m glad it was such a simple fix. Thanks for updating me!

i am scared of these tiny clear-ish thing in the water. I have 3 flag tail guppies i need help

Today we bought 4 koi fish while we are visiting someone and we live an hour from where our pond is.
One of the koi died after about 3 hours in a bucket. Can anyone help me with obout why it died and what caused it?? I need answers we payd alot and could loose all our spended money in a matter of a day.

Hi Ian- I don’t know where else to post this, so I’m putting it here. One of my tanks is being overrun with pond snails. I was trying to keep them under control by netting them out, and one weekend I must have gathered about 250 of them.. I felt it was probably stressing out my fish to keep going in there with a net, so I bought 3 assassin snails. I thought I noticed a difference, but now the pond snail population has exploded and I don’t see the assassin snails anywhere. My tank is heavily planted so it’s hard to see everywhere. Sometimes I think I’m missing a fish, then it appears the next day, so I think the plants hide them well. Is there anything I can use to get rid of them? I understand no Planaria nor panacur will eradicate them, but will harm the good snails. I’ve also read goldfish will eat them but my guppy tank is too warm for GF. Are there any fish that might eat them that get along well with guppies, or is there a medication that will clean up my tank? Thanks for your help. – yet again. Deborah

Hi Debroah,

An population explosion means that they have an abundance of food. Are you performing regular maintenance? Over feeding could also be the issue since any uneaten food becomes snail food.

In a properly maintained tank, assassin snails should be more than effective at keeping the population down. I’d look into where the food source is coming from and taking steps to rectify that.

my fish died and i cryed for the next 2 days he did the day after i put in new rocks “marbles” i cleaned them off and put them in first was the molly then 2 hours was the betta dose anyone know why he died?

My betta died after having her for one year, my poor little Rihanna was stressed with her new inhabitants, upgraded tank and plants that were not quarantined. After reading this I can definitely see where I went wrong. Buying guppies and just adding them in there with her, what was I thinking? I cried so much for 3 days and buried her in a cute little jewelry box i have, placed a gravestone from a candle lid and some flowers after I said a silent I’m sorry prayer. My family felt so bad for me. I have a new tank coming and am definitely following the cycling rule I was not truly aware of. I just let her sit in the new tank immediately! Oh if only I could go back in time, I have made peace with myself and learned from my mistakes as a beginner. Poor little Rihanna :”'(

Hi Marlenny,

I’m sorry to hear about the loss of your little Rihanna. It’s never easy saying goodbye. Don’t beat yourself up too much, you had the best of intentions and have learned from your mistakes. I’m sure your next fish will have an amazing life!

Im having the same problem i think. im trying to get my tank balanced with bacteria and to no avail. so tonight i bought 3 goldfish to use as test subjects but this was a very short test because they were all dead 3 hours later. i dont know what else to do besides start this whole tank from scratch.

Buy a better filter. I love aqua clear. And if ur tank isnt stable I would only and 1. The bacteria likely will have its work cut out for it with just one goldfish.

i bought 2 fishes one male and female ..they were two active after 2 days female fish body seems little black shade and frm stomach somethjng was coming ….four days got over that fish died…why really sad..

Hi Ian, I have a 28 litre tank which has been going for about a year. After starting with guppies and losing all but one gradually, I bought 6 neon tetras and all has been well for about 3 months. Last week I bought 3 ember tetras to add to the tank. I also bought a replacement filter cartridge and cleaned and replaced that after adding in the new fish. Within a day, 2 of the embers had died, so I went back to get another 5 to make the remaining one feel more secure. They didn’t have any so I bought a new wisteria plant instead. It was too tall so I trimmed off the top but it made a big mess when I put it is the tank – little bits everywhere! I think the neons were eating it. Around this time the last ember died. I couldn’t find any of the bodies but I have had guppy bodies I previously had not recovered so didn’t search too hard for them. The next night I found all but one of my neons were also dead. I checked the water – all the usual readings. I fished out all the dead neons then did a quarter water change and found 2 of the dead embers. Next day my last neon died, but my guppy seemed okay so I bought 8 more neons to keep him company. This morning my guppy is dead, new neons still swimming. Can’t work out the cause, was it the new fish, the plant debris, the filter change (gave all the plastic a good clean with tap water, and replaced both sponges, as I have done before)? I am worried for my new fish, and if they die, what action to take from there, do I need to empty and clean the tank and start again? Any advice would be appreciated, thanks!

Hi Ann,

I think you need to test your aquarium and read up on the nitrogen cycle as per reason one in the guide above. The results of your aquarium test kit will clue you in to what is happening in your tank, it’s almost impossible to keep fish without a test kit.

Thanks for your reply. I do test my water regularly with a kit I bought from the pet store. All the readings were as normal after the neons died. Anyway, thanks for replying, I will do another water check in a few days (going away for a few days, so my daughter is in charge). Maybe my new neons will survive and all will be okay! Thanks again!

Hi Ian, sorry for the late reply, we are away. Our ph was 8.2, ammonia was either 0 or 0.25 (I have trouble deciding this one), nitrite was 0 and nitrate was 5. These are the same readings I get every fortnight when I test the water. Well, sometimes nitrate is 0, and sometimes ammonia is clearly 0. Since my original comment, 5 of my new neons have died, only 3 left. I have just had my daughter test the water again, and she says ph is now still 8.2, ammonia is 0, nitrite 0 and nitrate is either 5 or 10, she’s not sure. Not looking good for the future of my tank!

Hi Ann,

That pH is very high for neon/ember tetras and is a possible cause of the problem. Generally speaking, tetra thrive in pH just less than 7. a pH of 8.2 is over 10 times more basic than their preferred level. There are products called “acid buffers” that are designed to lower pH levels, but if that is the pH of your water out of the tap, it might be a better option to consider fish better suited to that pH level.

Also, when you add new fish to your tank, do you drip acclimate them? Failing to do so can lead to death weeks or months later.

Hi Ian i purchased a 40L tank and 9 Guppies a few months later i have approx 40-50 guppies i didn’t realise they bred that quickly so i have now got a second tank 125L i cycled it for a week and have put 3 guppies in it yesterday but today they don’t look that happy just floating around the top of the tank in a little group, HELP should i put them back in the other tank? water test seems ok, I don’t want to kill them

Hi Vivien,

When you say you cycled the tank for a week, did you do it correctly using ammonia and test to ensure that ammonia and nitrite were at zero with elevated nitrates? I only ask as a week cycle is incredibly short unless you have taken specific measures to speed the cycle up. What does your water test return for ammonia, nitrite, nitrate and ph?

I got my fishtank yesterday. all the fishes seemed to fine but suddenly this morning on of the fish died .Also in the morning the others were swimming on the top surface of the tank I am not understanding the reason for it

Hi Ian – well, after struggling for 6 months I really thought I had everything under control, but I didn’t. I started losing several guppies almost daily. I had under 0.25ppm ammonia, but thought it was due to dead fish in the tank. I added prime and BB daily. I started dosing with maracyn but they kept dying. Too late, I moved the remaining 3 adults to an emergency tank, it’s been 2 weeks and they are still alive and doing well. Interestingly, I had 10 8-wk old fry in that tank and none of them died. The fry are also still alive and growing. Something about that tank killed them. My water source is the same for all my tanks, I didn’t treat that tank any different than my others, and my other tanks are doing fine, no problems. I do 25-30% WC every 5 days. pH was 7, 0 nitrates/nitrites. I’m totally breaking the tank down, will sanitize with a bleach solution. I’m switching from gravel to sand. My question is, can I soak the HOB filter, the sponge filter and heater in a bleach solution? I don’t know if plastic will absorb the bleach or not. Or is really hot water enough? I’ve previously tried soaking plants in a bleach solution for a different tank and all the plants died. Is hydrogen peroxide an option? I’m so paranoid right now. I’m going to fishless cycle per your instructions. Can I add plants before the tank is cycled or should I wait until after. I’m thinking about soaking all plants in ParaGuard or antibiotics or something similar that will kill everything, before I plant them, so I won’t have to worry about harming the fish. I’m at a loss about what happened. I’m not expecting you to figure it out, but I just want to know the best way to clean the equipment and remaining plants. Or should I just throw the plants out and get new ones? I’m putting my 2nd 20G and55G on hold till this one is up and running again. The one thing I’m thinking is this is the only tank that had a 3D background. I was unable to get a vacuum behind it to clean because there wasn’t enough room to get it back there. Could enough junk and dirt build up polluting the water? I haven’t emptied it because I haven’t been up to it yet, so I don’t know what it looks like behind it. I lost 15 guppies. Thanks for your help. Here’s to starting over. I feel so bad for those innocent little guppies.

Hi Deborah,

Sorry for the delayed reply, the messaging system on the backend screwed up and everything came through at once today.

I’m so sorry to hear about your misfortunes. It sounds like a mystery as to what happened in this tank, especially if it tested normally and it’s the same as your others.

A h202 and hotwater solution is totally fine. Both bleach and h202 run the risk of discoloring, so be careful here. You can even use hotwater and soap, but if you do, make sure you rinse incredibly thoroughly. Sponge filter is best replaced rather than cleaned.

H202 dip is my preferred option although bleach works well. Be mindful that it’s a dip rather than a soak. Also, leaves only and be mindful that some plants need a really low concentration, so dilute if necessary – for example, B. Japonica will melt in all but really low concentrations.

Plants can absolutely be added before a tank is cycled. In fact, I recommend it since it’s easier to scape a tank as you are adding water rather than after.

If I was in your shoes, I would try and save the plants. I certainly wouldn’t soak them in fish medication. A h2o2 dip should be fine.

If the 3d background is the only thing different, then it is certainly possible that is the cause. Depending on the quality, it could also be leeching chemicals (in the past there have been some dodgy chinese backgrounds going around) but that is just a theory.

Hang in there, I know it sucks starting again but look on the bright side – this is an opportunity to set up a tank in a new style that you might have not otherwise had the opportunity to do

Wishing you all the best Deborah!

Thanks for the support, Ian. That’s where I went wrong with the other plants I tried to disinfect. I soaked the roots in the bleach solution. I’m glad I can dip and still use these plants. And you’re right – I have the chance to scape differently- for one, I’m switching to sand. And I can move the tank as well. I have noticed the 3D background is changing colors in some areas. I bought this from Universal Rocks in Texas. On a weird note: I have 3 Fluval 5G tanks for fry, shrimp, sick /quarantine use. My pregnant red albino was in one tank about to have babies. I don’t know how many she had but I got up one morning and there was only one in the tank. This was around Christmas. After a day or so, that baby was gone, I put the female back in the main tank. I unplugged the heater and filter. Earlier this week I started to break it down (it’s more fun putting them up than breaking them down) and saw a small fish swimming in the filter area which is separated from the main tank! I got him out, slowly acclimated him to the fry tank adding fry tank water to a cup he was in with his tank water, leaving him in the cup for about 30 minutes until both waters were the same temperature. How he survived with no filter, heat, food, WC for about 2 months is beyond me. He’s small for his age but he’s still alive, although it hasn’t been long enough for me to feel comfortable that he’s going to make it. He’s all I have of the red albinos since both adults died in that mess of what happened. Strange…the stuff you work at fails, the stuff you do nothing with succeeds…..

I know this has been a horrible experience but there are lots of positives here, especially discovering the baby in the filter – I have my fingers crossed that he pulls through.

Oh, make sure you read up on sand beds, it’s very different to a gravel/rock or soil substrate. They need to be stirred every so often. A quick tip for using a gravel cleaner here – tape a chopstick to the side of the tube so it protrudes out the end. This way the chopstick stirs up the sand and any poop and other stuff floats up towards the vac. Gravel vacuuming the sand directly will suck it up and into your bucket, because it’s so lightweight, so the chopstick on the end also gives the tube distance from the sand. You’ll need to experiment to find the right length to stick it out.

Thanks as always for the updates Deborah!

Hi Ian! I stumbled on your blog and have learned a great deal from it. Thanks for sharing your knowledge with a noobie like me.

We started out with a gold fish for my three year old son and things went great for a while. Until they didn’t. The water never clouded up like it has for our new tank (more on that in a minute). My husband bought a “friend” for Boxy the goldfish, Lulu. Regrettably we lost both fish soon after. I think that was probably because they were in a 5 gallon tank which was too small for two fish.

Grandpa decided it’d be a good idea to get him a GloFish tank for his birthday as my son was kind of sad that his fish died. Once again we got the tank up and running, excited to add new fish. Unfortunately I made the newbie mistake of only letting the filter run for 24 hours before adding the new fish. They lasted for a while but after a month or so, they started dying off. 🙁

While the poor fish were stressed out of their minds, we started having issues with cloudy water. Water Clearer would help but after we stopped using it, the foggy water would come back. Then we tried adding a Plecko and some plants. Well the poor Plecko died and we found his bones being picked at by our GloShark. The plants of course didn’t make it either. All the decayed tissues and plants of course exacerbated an already skewed ecosystem.

I tried using a new filter… the AquaClear with the carbon bag, sponge and “bio bag”. Well this made the water Green Aquarium water that looks like pea soup. I went to the pet store today and they gave me Algaefix but after readying your blog about Green Aquarium water, I ordered a UV sanitizer light and will wait for that attempt before I start adding chemicals.

I also bought the test tube water testing kit as the strips aren’t as accurate, per another one of your awesome blog posts. I can attest that this is correct. The strips made it seem like only the KH was a little low. However, the test tubes showed a very different story.

My pH is at 8.0 which is horrible for Tetras. The ammonia levels are too high as well… around 0.25- 0.50 ppm give or take. I will say that at least the Nitrite and Nitrate levels are good… Nitrites at 0 ppm and Nitrates at 10-20 ppm give or take.

I am concerned about the pH and the ammonia. I know that the ammonia could be due to the new filter but I’m not sure how long that lasts? I added the new filter a month ago (Feb. 2). We have cut back on how much they are fed. I think I mentioned but am not sure… we have two GloTetras in a 10 gallon tank.

Is there anything I can do to get the pH under control? I use the Aqueon water conditioner and API Quick Start whenever I change the water. This was 10% once a week. However, I tried letting it go for two weeks to see if that would help with the cloudy water. Of course this didn’t. I also use Tetra WaterClarifier when changing the water. Also, is Seachem Stability the same as API Quick Start? Better? Worse?

Thanks in advance for your time!

Hi Alexis,

Firstly, welcome to the hobby. I know this seems like a whole lot to take in but trust me, it definitely gets easier.

I think the problems have come from the zig-zag approach you have taken – ideally you want the strips first so you know exactly what is happening with your tank.

The first thing you should do is test your tap water. Pour a glass, add some dechlorinator and leave it to sit for and hour. Then run your ph, kh/gh ammonia, nitrite and nitrate test. This will determine the quality of water you are dealing with. Typically, it’s best to choose fish that match this water. For instance, cichlids prefer hard water. By buying a fish that matches your water, you won’t have to by chemicals and additives to balance the ph/kh on an ongoing basis (it’s much cheaper and less work)

With that out of the way, what you want to do now is perform a fish-in cycle, as per this guide.

This will get your ammonia levels down. It will take some weeks (or even months) but it’s an important step to keeping all future fish safe.

On the seachem stability vs quickstart, these are products that supposedly contain live bacteria. I don’t really believe in them since I have seen tanks cycle as quick or as slow without these, they do not appear to be responsible for speeding up tanks. Since its an added cost, and fish keeping can add up, I don’t recommend them.

On the algae, do you run your lights 24/7? If so, it can kickstart algae growth and stress out your fish. One of those plug-in timers will allow you to schedule the lights. Typically 8-12 hours/day is more than enough. Fish need their darkness too!

Hi, I love your blog. So my question is I had an Oranda and pearlscale in 55 gallon for 6 months and seemed to be thriving. I also had a betta in 10gallon. I also have a 20 gallon with glo tetras that are doing great. So I purchased 3 more fancy goldfish online that was shipped. When they arrived I made the mistake of not quarantine them. So the new pearlscale couldn’t swim but now is swimming a little but unable to eat. A few days later I noticed ich on the fish. So I treated. Now only one fish still alive in the tank and he’s not looking to good. Now I’m also seeing what seems to look like worms possibly in the tank. My betta in a different tank also got ich. So I think I must have cross contaminatied. My question is do I need to restart or can I wait a certain amount of time before adding new fish?

Hi Stephanie,

Thanks for the kind words! This guide should help you understand ich and it’s lifecycle:

Essentially, if the tank is empty, you can raise the temperature and wait. Ich is a parasite that needs a food source. Without food ich will starve and once it completely dies off, you should be good to add your new fish. If it’s in a tank with other fish, I’d recommend following the removal instructions in that guide.

Hoping you can help, I have a 20 gal planted tank with 10 mollies, platties and sword tail. Plants are water sprite, sword, banana plants. The fish have been doing fine for months and looking healthy. All of a sudden I have one black molly that was looking fat and healthy and within a few days it lost weight, was sitting on bottom of tank and looked like it had a bent backbone. within two or three days it was dead. I have since lost two more. The tank has been set up for about 9 months and I do regular water changes, The ammonia is 0, the nitrite is 0, nitrate is 5ppm, ph is 7.4-7.6, temp is 78deg, everything looks good but not sure why these guys are dying. I don’t know what else to check for. thoughts?

Hi James,

There are a bunch of diseases that have a bent backbone as a symptom Including fish tb. If your water seems fine, you should be trying to match the symptoms with a disease. You should also be testing the water daily, and monitoring changes, since pH swings and the like can also cause issues.

why are fishes dieing, we have a pregnant guppy( which is dieing), swordtail. platy and danios. We have done a water change, put in good bactetia, ph down solution, and a bit of aquarium salt. Have done water checks, nitrite is 0.25, ph low range 6.6, nitrate good, and ammonia good. Dont know what i am doing wrong, hate to see my fish dieing, and not knowing what i am doing wrong.
I have a 94 litre tank. Any advice would be appreciated.

Hi Kim,

Go through this guide. A measurable nitrite suggests your tank isn’t cycled. it should read zero.

I have guppies 2 goldfish 2angels 4 fantails,2 mollies I lost all my fantailes 1goldfish 1guppie and both mollies,what is the reason for them dying,I did a water change and then the last mollie died

My Molly died 2 hrs before and because I used to chase fishes in the pot ,often remove them from water and putting them back afterwards .I had 2 fishes including tiger Barb and the actual reason why it died is because I hold its tail and removed from water and then it jumped and fell on floor and it happened 2 times and I know it was painful for fish to get so much pain ,the floor was dry and it was too hard for it to get beat up from height and it died afterwards .I cried for an hour because it died because of me

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