Last update: March 15, 2024

The danger of aquarium NITRITES to fish (and how to get rid of them!)

Alright, let’s get real for a second—nitrites are like that one uninvited guest at your aquarium party. I don’t fancy them, you’re not a fan, and your fish? They’re definitely not throwing any welcome parades.

But here’s the twist: as much as we all want to show nitrites the door, they’ve actually got a VIP pass to the ecosystem bash in your tank. I know, it sounds crazy, but stick with me.

Today, I’m on a mission to unravel the mystery of nitrites for you. And trust me, by the end of this, you’ll see them in a whole new light.

Now, before we dive deep, do yourself a favor and check out The Nitrogen Cycle guide. It’s like the pre-game show that gets you all warmed up for the main event. Especially if you’re new to the aquarium scene, it’s a game-changer.

Let’s roll out the red carpet and really get into the nitty-gritty of nitrites.

Before I continue, I want to make sure you read The Nitrogen Cycle guide.

Up to speed?


Then, let’s continue…

Also, check out these must-have gadgets for optimal water control and checkups

What are aquarium nitrites and where do they come from?

Pronounced nigh-trights, nitrites (NO2) are a form of dissolved nitrogen that occur naturally in the water column, which is the water inside your aquarium.[1]

How nitrites end up in your aquarium is quite a journey.

And, it all starts with waste.

Without waste, nitrites wouldn’t exist.

Common sources of waste found in aquariums include:

  • Uneaten fish food
  • Fish poop
  • Decaying plants
  • Dirty filters

While waste leads to the appearance of nitrites, it is not the defining cause.

To explain, here is a very brief recap of the nitrogen cycle.

Waste breaking down into ammonia nitrite and nitrate in aquarium diagram

1. As waste breaks down, it gives off ammonia.

2. A bacteria called nitrosomonas breaks down the ammonia into nitrites.

3. A second bacteria, called nitrobacter, then feeds on the nitrites and produces nitrates.

So, nitrites wouldn’t exist without ammonia.

Ammonia converted to nitrite by bacteria diagram

The process of ammonia being converted into nitrites by bacteria is called nitrification.

What nitrite levels are acceptable in your aquarium?

The acceptable level of nitrites in your tank is…


Yep, you read that right…

Nitrites are pretty bad news. So, in a stocked tank, the level should read 0 ppm (parts per million).

The bacteria in an established tank should exist in high enough numbers to break down nitrites the moment they are produced – leading to a zero reading when tested.

If your nitrite levels are higher than zero, you need to find the cause of the problem, and fast.

However, there is an obvious exception to this rule…

When cycling a new tank.

In a new tank, bacteria doesn’t exist in large enough numbers to break down the nitrites as they are produced.

This is actually a big part of why you need to cycle your aquarium – to allow the bacteria that live in your biological filter, which break down nitrites, to grow.

So if you are cycling a new tank, expect high levels of nitrites.

What happens if your aquarium nitrite levels are too high?

Once nitrite levels exceed zero, they are going to be absorbed by your fish – they have no choice in the matter.

When the nitrites reach the fish’s blood stream, they stop the blood from carrying oxygen. The more nitrites that are taken in by your fish, the less oxygen the blood can carry.[4]

Oxygen is kind of a big deal because your fish need it to live.

So, I am sure you know what happens if your fish take in too many nitrites, right?


What a horrible way to go.

What’s interesting is that it doesn’t matter how oxygenated your aquarium is, your fish can still suffocate.

An aquarium with all the aquarium bubblers and surface agitation in the world isn’t going to help your fish if they cannot draw oxygen out of the water.

This entire process is referred to as nitrite poisoning.

Important: Don’t confuse nitrites with nitrates. While there may only be one letter differentiating them, the two are very different.

The toxicity of nitrites depends entirely on the species of fish. While some are quite tolerant, others will die at the faintest hint. For some fish, just 0.29 ppm of nitrites leads to death.[4]

But just because your fish can tolerate it, that doesn’t mean it is healthy. Your fish are likely under an incredible amount of stress….

When fish are stressed, they are more likely to develop diseases, lose their color, suffer from stunted growth or even become unable to reproduce.[4]

It’s simple – keep your nitrite levels at zero for happy and healthy fish.

FishLab Fact: Chloride reduces how many nitrites your fish take in. And, you know what has lots of chloride in it? Salt. For this reason, marine fish are more tolerant of nitrites than freshwater fish.

What causes high nitrite levels in aquariums?

Does your tank feature any of the following?

  • Over feeding
  • A sudden buildup of waste
  • No nitrifying bacteria in the filter
  • Weak filtration
  • Overstocked aquarium

If so, your tank is at a high risk of elevated nitrite levels.

FishLab Fact: A sudden jump in nitrite levels is called a nitrite spike.

Also, if there is a sharp rise in ammonia levels, a sharp rise in nitrites will soon follow.


The bacteria responsible for turning ammonia into nitrites rapidly multiply because their food source has become bigger – releasing more nitrites than your tank can handle.

As you may have noticed, most of the causes of high nitrite levels can be fixed with good housekeeping and regular maintenance.

How do you test for nitrites?

Nitrite testers are readily available at pet shops, fish stores and even online.

Let’s take a closer look at some of the most popular nitrite testers on the market.

API Nitrite Test Kit

61Ja7 HZMGL. AC SX679

Lasts up to 180 tests

When it comes to measuring your nitrite levels, an aquarium test kit is the simplest and cheapest way to do it.

If you walk into your local fish store, this will likely be the nitrite test kit that they keep on the shelf. API test kits can be found almost anywhere.

That’s not a bad thing. API nitrite test kits are cheap, and they work well enough. For the average hobbyist, these are fine.

If you purchased the API Master Test Kit, then you will already have this test kit, along with many others.

However, like many nitrite test kits, this one has a shortcoming.

It’s all thanks to amine interference.

In certain instances, such as reef tanks, essential amino acids can cause these tests to show a lower nitrite level than what is actually occurring in your tank.

And, that could prove to be deadly.

Fortunately, there are nitrite tests available that get around this issue.

Unfortunately, many of these test kits are expensive.

But not this one…

API NITRITE TEST KIT 180-Test Freshwater and Saltwater Aquarium Test Kit
  • Contains one (1) API NITRITE TEST KIT 180-Test Freshwater and Saltwater Aquarium Test Kit, including 1 bottle of testing solution, 1 color card and 1 test tube with cap
  • Helps monitor nitrite and prevent invisible water problems that can be harmful to fish
  • Accurately detects high nitrate levels from 0-5 ppm

Salifert Nitrite NO3 Test Kit

Lasts up to 60 tests

This nitrite test kit hits the sweet spot for both price and performance.

It may not last as long as the API test kit, but it makes up for this by being easy to use with accurate results – no amine interference here!

The Salifert nitrite test kit is particularly popular with saltwater tank owners.[1]

I personally use Salifert because I find these kits easier to use.

Since it can be picked up for just a few more dollars than the API kit, I recommend making this your go-to nitrite test kit.

While nitrite test kits are easy to use, their accuracy depends on your ability to follow the instructions.

I highly recommend reading the instruction manual thoroughly before using a test kit. Skipping a simple step, such as shaking the bottle, can cause an incorrect reading.

Salifert NIPT Nitrite Test Kit
  • Because of its unique formulation, the Salifert Test Kit does not suffer from amine interference, and offers full color development within one minute
  • The test kit has two different scales: an ultra-low scale for nitrite-nitrogen which ranges from 0.002 – 0.12 mg/L, and a medium scale for nitrite-nitrogen which ranges from 0.02 – 1.2 mg/L
  • Sufficient for up 50 tests

Hanna Nitrite Colorimeter

512l5veYmYL. AC SX679

If you want to spend a bit more cash for the sake of accuracy, Hanna Instruments’ nitrite checker bridges the gap between the cheaper chemical test kits and the more pricey probe meters.

Simply add your water sample, combine with the reagent from the kit, and the checker will give you a digital readout.

While the checker comes with some reagent, you will burn through it quickly. Refill packs are readily available, but they will be an ongoing cost.

Just make sure you get the correct reader for your aquarium – the freshwater nitrite monitor can give an incorrect reading if used with saltwater.

Hanna Instruments HI782-25 Marine Nitrate High Range Checker Reagents (25 Tests)
  • Pre-made reagents for ease of use
  • Prepared with high purity chemicals
  • Marked with expiration date and lot number for traceability

How do you reduce nitrite levels?

Just tested your nitrite levels and discovered they are shockingly high?

Don’t panic!

I am going to talk you through everything you need to do in the event of a nitrite spike.

1. Water change!

A 30-50% water change should be the first thing you do after confirming a nitrite spike.

You are swapping out nitrite-filled water for water that is nitrite-free.

What you are essentially doing is diluting the amount of nitrites that are present in your aquarium.

Performing a water change will buy time until you can use another method listed below to get your nitrites under control.

2. Add cycled filters

As I touched on earlier, bacteria turn nitrites into much less harmful nitrates.

In large enough numbers, these bacteria eat nitrites as fast as they are produced.

So, what you are doing is introducing more of them into your aquarium.

You see, in a cycled tank, these bacteria are present in large numbers in the filter.

By adding a filter from a cycled tank into an aquarium with a nitrite spike, the bacteria start eating all the nitrites, lowering the levels.

But if you don’t have multiple tanks, tracking down a cycled filter can be difficult. If you have a good relationship with your local fish store, they may allow you to take one.

FishLab Tip:If you have more than one tank, use a sponge filter to cycle multiple filters at once. If a cycled filter is needed, simply remove a sponge filter from one tank and add it to another.

3. Water conditioner

This is essentially a nitrite remover in a bottle. Simply follow the directions and watch the nitrites disappear from your aquarium as if by magic.

Water conditioners bind the nitrites, thus rendering them harmless to fish and giving your filter bacteria the opportunity to catch up and turn them into nitrates.

I make it no secret that my favorite water conditioner is Seachem Prime


Not only does it detoxify nitrites, but it also deals with nearly any other water quality issue that may arise![2]

I highly recommend keeping a bottle on hand in case of an emergency.

And trust me, a nitrite spike definitely classifies as an emergency.

While these methods help reduce nitrite levels, they do not stop the cause of the spikes.

With the nitrite levels under control for the moment, you now have time to track down the cause of the problem.

How do you prevent nitrites from building up?

If you read my guides on ammonia and nitrates, you are aware of the advice I am about to give you.

That’s because ammonia, nitrite and nitrate share a common cause.

It all starts with waste.

By getting waste under control, you minimize any problems caused further down the nitrogen cycle.

While these tips won’t remove or reduce existing nitrites in your aquarium, they will stop new nitrites from building up as quickly – which is a good thing!

1. Don’t overfeed your fish!

The more your fish eat, the more they are going to poop. And, the more they poop, the faster the nitrites will build up inside your aquarium.

I would also add that any food that remains uneaten sinks to the bottom and rots. And, you know what that means? More nitrites and nitrates!

Remember, just because you don’t overfeed your fish doesn’t mean that someone else in your family isn’t.

I learned this the hard way…

I kept finding uneaten pieces of food rotting in the corner of my tank. I feed my fish a premeasured amount, and they eat every last piece. But where this extra food was coming from had me stumped.

Well, it turned out my nephew was sneaking in and feeding the fish when I wasn’t looking. Mystery solved.

2. Maintain your plants

If you have live plants in your tank, then you need to incorporate a bit of underwater gardening.

Old leaves on your plant are going to die.

When this happens, the leaves fall off and decay, which can lead to an increase in nitrite levels.

You can get ahead of this by doing a bit of light pruning.

Every time you see dead or dying bits of plant matter, snip them off and remove them from your aquarium.

3. Clean your filter

You know all those loose bits of food, plant matter and poop? Well, they are going to get sucked up and trapped by your filter.

Over time, this waste is going to accumulate.

Regular maintenance will stop the built-up waste from reaching problem levels.

One popular solution is to use an intake sponge filter – literally a sponge that covers the intake to your filter.

The sponge traps any gunk that floats through the water, trapping it before it reaches your filter.

Simply remove the sponge to clean it – no more taking your filter apart!

4. Buy a gravel vac

Your substrate can quickly become a graveyard of rotting fish food, dead plants and whatever else that gets caught up in it.

All of these contribute to nitrites forming in the water.

A gravel vac is one of the most effective ways to remove this from your substrate.

Easy to use, affordable and effective – if your tank has a substrate, there is little excuse for not owning a gravel vac.

Ideally, you should perform a gravel vac during most, if not all, water changes.

5. Monitor your stock

An overstocked aquarium will have more nitrite problems than an understocked one.

It’s unavoidable. Too many fish in a small aquarium will create more waste than the tank can handle, which is a surefire recipe for a spike in nitrite levels.

If your tank is overstocked, you have two solutions:

  • Get rid of some of your fish or
  • Buy a larger aquarium.

I know it can be difficult to say goodbye to fish, but it’s for their own good. They are just going to suffer in an overstocked tank anyway.

Or, if you don’t want to say goodbye to your fishy friends just yet, buy a bigger tank.

You have been looking for a good excuse to buy a larger aquarium, and nitrite problems are as good a reason as any to make the upgrade!

Don’t be tempted to buy more fish now that you have a larger tank – that’s what got you into this whole mess in the first place.

6. Regularly Test Ammonia, Nitrite, Nitrate, and KH Chemistry Levels

Regular checks for ammonia, nitrite, nitrate, and KH chemistry in your tank are indispensable for fish well-being. High levels of ammonia and nitrite are particularly hazardous, causing health issues that can be avoided through early detection from routine tests. KH chemistry, or carbonate hardness, is another essential parameter, as it acts as a buffer against sudden pH changes. I’d suggest regular testing to catch any imbalances early on.

7. Keep an Eye on pH, KH Chemistry, and Temperature

Stable pH and temperature are vital, and KH chemistry plays an influential role in maintaining a stable pH level. Fluctuations in any of these parameters can stress your fish. I would recommend regularly monitoring and adjusting these factors to establish a balanced and thriving habitat for your fish.

Quarantine New Additions

When you add new fish, they should be quarantined for careful observation and acclimation, ensuring the well-being of both the newcomers and your established aquatic residents.

Setting Up a Quarantine Tank

It’s crucial to set up a quarantine tank with proper filtration and appropriate conditions, including monitoring KH chemistry. In a controlled environment, you can spot early signs of stress or disease, acting as a safeguard for your main tank.

Importance of Quarantine Procedures

These procedures are foundational for responsible fishkeeping. They mitigate the risk of spreading diseases or parasites that could devastate your tank. I firmly believe this practice is a cornerstone for maintaining a thriving aquatic ecosystem.


In fishkeeping, vigilance and proactivity can’t be overstated. Elevated levels of nitrite or imbalances in KH chemistry could potentially lead to an array of health issues. Regularly monitoring essential water parameters, such as ammonia, nitrite, nitrate, pH, KH chemistry, and temperature, is a fundamental practice that helps you spot and address any irregularities promptly.

By adhering to strict quarantine procedures when introducing new fish, you not only protect your existing community but also enhance the overall health and longevity of your aquarium.

Do you have any nitrite tips? 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 (81)

very good article. thank you. I am an old acquariast. Back in the day, I had two 55 gallon tanks. One for Discus, and one for Angel Fish. I have been away from the hobby for 10 years, but never lost my interest in the hobby. A month ago, my wife decided to order a 2.5 gallon fish bowl, for a betta fish. Guess who now is taking care of it ? However she purchased the beta before cycling the tank. So I am cycling it the hard way, with the fish in it. Its been about 6 weeks , and it is almost cycled. Not an easy thing to cycle a 2.5 gallon fish bowl.

So now, I purchased last week a second 2.5 gallon. I am doing a fishles cycling of the tank, using ammonium chloride. As of this morning, I had 2ppm of ammonia, 2 ppm of nitrite, and 4.00 Nitrate. So the cycle is well on its way. I have added a full 2 oz bottle of Dr. Tims One and Only Nitrifying bacteriia, also, as an experiment. Will let you know how it all goes.

Hi Irving!

Ohhh, poor little betta – Fish-in cycles can be rough. I hope it survives the cycling process! Would love to hear an update when you are done. Fingers crossed it all goes well 🙂

Bettas should be kept in a 10g tank minimum… imagine if we stuffed you inside a VW bug to live out your life?

Hi Rob,

Not a good comparison. That’s apples to oranges. While I agree bigger is better, the general consensus among the fish keeping community is 2.5 gallon at a minimum although I generally recommend 5g as a starting point. The debate of the “right sized tank” for a betta will never be settled and I’m certainly not looking to argue it in the comments section of a website.

The best information I could find. This has been very helpful. I’m new at this and I get so frustrated sometimes. I just love having the aquariums. There is a lot to learn. I have both a freshwater and marine tank and they keep me busy.
Thank you so much for the information that I can understand.

I am new to this so researching and reading blogs about this helps me a lot.
I have started with uncycled fish tank then started to have ammonia rise after treating it I have been 0 ammonia for couple of days but my Nitrite has spike. Do I have to continue with every day water change? The same I treated the ammonia spike?

Thank you.

Dear Ian,
This has helped me considerably. Even though I have read all manner of Blog/Tech information on cycling its only after a years experience and your information ( I’m in a nitrite crisis right now) that I have a better understanding of how to manage nitrates and the results. It also helps to explore fully the options available in Juwel filters!
Great stuff. Thank you.

Hi Gavin,

Thank you for the lovely feedback – it’s actually why I created this blog. Despite there being so much information on the topic, beginners were still making mistake.

It can be difficult to get your head around at first, but keep at it. It definitely gets easier. Good luck 🙂

I have e a 40 gallon aquarium….have no spacece or money for a bigger one…I also have 2 20 gal n 1 15..I will not give my fish away…I could part with a few when they’re bigger…I do lots of water changes and use prime pristine and stability to keep my ammonia and nitrites in check if my tanks mini cycle..why does no one ever recommend handling it This way …it’s always GET A BIGGER TANK …OT…GET RID OF FISH. a tank can be a little overstocked if handled in this matter

Hi Janie,

The reason for this recommendation is simple: Most people, especially beginners, barely keep on top of their water quality and chemistry as it is.

My guides are aimed at beginners and so is the advice. As a result, those are the best two options.

Besides, whether it’s a little or a lot, an overstocked tank is an overstocked tank. It’s not something to be desired and definitely not something to be encouraged.

This doctor of chemistry from Harvard states that Nitrites for marine aquariums have to be VERY high to cause damage or stress to fish not zero. Obviously zero is the goal but you can absolutely start stocking a tank with > zero nitrites as long as your ammonia levels are very low.

Hi Ron,

I see what you are saying. My site primarily focuses on freshwater tanks for beginners. All cycling guides, algae, disease etc. on my site that use this guide as reference focus on freshwater tanks. In freshwater tanks, it’s accepted that nitrite is an extreme stressor and zero ppm is the aim. Since my goal is to educate beginners, I have tried not to complicate the topic by introducing variables – water chemistry is already difficult enough for a newcomer to grasp.

Since I don’t have a saltwater section (yet) all guides are created with freshwater tanks in mind. As I expand the site, saltwater will get it’s own water chemistry section. And yes, I completely agree with Randy’s findings. He has done amazing work in advancing the saltwater side of the hobby.

I do thank you for your concern and I will make a note that I need to make it more clear that this is a freshwater nitrite guide rather than a universal guide.

No harm done at all Ron and there is definitely no need to apologize. I appreciate really appreciate your feedback. You have made me realize that I need to make it clearer that information is for freshwater, so I don’t confuse any saltwater tank owners. I’ll speak to my website developer about the best way to do this.

All the best!


I’m in the process of cycling (fishless cycle) my 30 gallon tank. I have just completed a water test for ammonia, nitrites, and nitrates. My ammonia is at 1.0ppm, my nitrites are at 5.0ppm, and my nitrates are at around 30ppm. Should I do a water change to “dilute” the amount of nitrites? Or is this ok to just wait out?


Hi Megan,

It’s entirely up to you. My preference is to do a water change to lower the nitrites. I assume you are using the API nitrite test? The upper limit on the card is 5ppm. This way you can see if your nitrite levels are continuing to rise or are beginning to lower, which gives you a better understanding of what is happening in your tank. Alternatively, you can add a 50-50 mix of distilled water and tank water to your tube when taking a test. This will halve the reading – double this to get the actual result. However, a water change and sticking to the card leaves less room for error.

Very helpful post! I just started a new tank about 3 weeks ago and was doing really well with my nitrites, has positive nitrates so I knew my tank was starting to cycle. How long does cycling a new tank take? I added a new plant, and I feel my nitrites have gone up since. The plant doesn’t look dead at all, but the bottom does have some fuzzy algae stuff. Would this maybe be a cause for my higher nitrites? Should I remove the plant and see how it does? I have been doing 50% water changes the last three days, and nitrites are still high. Could I buy some of that conditioner you recommended or would that stunt my new tank cycling? Help! Thanks in advance.

Hi Megan,

I recommend reading up on my fishless cycling guide for more info on what to expect when cycling your tank, and how the best way to go about it. If you have a fish already, check out my fish-in cycle guide instead.

As for the stuff on your plant, it’s possibly just bacteria – this commonly forms on newly added driftwood or recently purchased plants. It normally goes away on it’s own, but you can wipe it off if you don’t like the look of it.

Thanks for the article. Very useful and easy to understand. I’m new to this hobby and two weeks back I bought 75 gallon tank for salt water fish. I had no idea about water cycling and add 14 fish and few corals in it right away. I’m learning the lesson hard way. Corals never came out since I add them last week. Probably they are dying. I already lost two fish. Two days ago I add Bio Spira beneficial bacteria to the tank. Luckily today ammonia level came down to 0.25 ppm but Nitrite rise up to 5.0 ppm. Hopefully tank will complete the cycle within next few days….Once again thanks for your article.

Hi Dan,

Welcome to the hobby. There is a lot to learn but don’t be put off, it’s very rewarding once you get your head around it!

For saltwater tanks, I would suggest reading up on “live rock.” in my opinion it’s the best way to cycle a saltwater tank.

I have an established tank for three months just a small 10 gallon with live sand, LR 2 clown fish 1 condy anemonme and a sponge coral.
Everything was fine until I decided to try to clean the sand and start it all up. Now I have a spike in my tank!!
0.25 nitrite
0.25 ammonia
20 ppm nitrate
I did a 30% water change but didn’t help.
wish I never stirred the sand!
any suggestions? also how long will this spike last? is it like cycling a tank all over again??

Hi Cathy,

The advice on this tank mostly pertains to freshwater fish. However, when you stirred your sand, it’s possible you released pockets of nitrogen gas and other gunk that was trapped. I recommend reaching out to a reef forum like “reef2reef” or “reefcentral” they will be able to point you in the right direction.

Hi Ian,
I’m having a bit of a nitrite crisis, I recently moved my two fancy goldfish into a larger tank. I had the new tank setup and cycling with an old filter (which broke) just after the tank cycled, all levels were 0 (Nitrates were about 5.0ppm) I added the biological media from the old filter into the new external filter, and the next day moved the fish over to the new tank also. This was 5 days ago and this evening I noticed one of my fish was surface gulping a bit, after testing the water I found nitrites were 1.0ppm, ammonia was about 1.5ppm and nitrate 20ppm. This is after doing a 30% water change this morning.
What do you think caused the spike, I did have a live plant in there that I added before the fish but took out because they had almost stripped it bare in a matter of days (so much so I had to remove it and use a net to remove a lot of debris) I now only have a plastic plant. I did another 25% water change this evening after testing, added Prime and a small amount of api aquarium salt. I’m thinking it’s maybe a combination of the new filter and extra waste from them eating the live plants.
I feel terrible and hope they will get through this, am I doing right by continuing with the daily water changes, should I be testing the water daily too to check the levels are coming down?
Sorry for the long post, I wanted to include as much detail as possible.

Hi Louise,

If ever you notice spikes, daily testing and water changes in relation to the test results is a must.

Did you confirm that the new tank was able to process 4 ppm of ammonia before adding the fish?

Hi Ian
No I didn’t, I’m fairly new to the hobby so I’m afraid a few mistakes may have been made. I will keep testing and doing daily water changes though for the time being.

Hi Lousie,

Not a problem, we all make blunders. I made some huge mistakes when I first got into this hobby. Don’t worry, it all get’s easier from here. Hang int here!

The reason 4 ppm of ammonia is added before you add your fish is to check to see that the beneficial bacteria that have colonized your filter are in a high enough number to process the ammonia into nitrate in a 24 hour period. If it can do this, it can do the same for a appropriately stocked fish tank. If it can’t, you’ll know your tank isn’t cycled yet and is still unsafe for fish.

How long will it take added bacteria like nite out or special blend to work? I did a 50 percent water change and added stuff yesterday but am still only seeing slightly better test results. Am I just impatient or should I do another major water change?

Hi Julie,

I would suggest reading up on my fishless cycle guide – the short answer is that cycling a tank can take a few days, a few weeks or a few months. There is no hard and fast answer here. Patience is certainly key.

Sadly this is not a fishless tank but rather a tank with 150 rainbow trout in the fry stage. The tank has been set up since December for this classroom science project. Yesterday we took out 19 dead fish :(.

Hi Julie,

Ordinarily, you would use a fish-in cycle then. In the hobby, rainbow trout are pond fish. Being general here, you would need half an acre of pond surface area and a depth of 8 feet for 150 fish. Admittedly this calculation uses adult fish, which grow quite large, but it should give you an idea of how unsuited to aquariums rainbow trout are.

In a tank, you would ordinarily use a fish-in cycle. However, fry being particularly sensitive to ammonia and nitrite often don’t make it. Rainbow trout are not good aquarium fish and require highly specialized aquaculture setups, where they are grown for food. Unfortunately, in a small tank, most if not all of the rainbow trout will very likely die. Oxygen deprivation, elevated ammonia, nitrite and nitrate levels are both of concern here.

Thank you for the information. We had a nitrite spike to about 10ppm which may have been caused by uneaten algae which we put in for our Oto (the tank is very clean so we were concerned he wasn’t getting enough food). One guppy died before we realized the problem. We did a 50% water change over a 24 hr period. 2 days later the nitrites are still near 10ppm. Is it best to replace our monthly filter (it is due for replacing) or rather leave it for a while in case it has some of the good bacteria that we need to help with the nitrites? Thank you from an aquarium novice.

Hi Noelene,

Are you talking about disposable filter cartridges? ‘Cause if so, those are a bit of scam. They actually cause you to throw out your beneficial bacteria each time, which can cause a tank crash.The sponge in my current filter has lasted almost a year and the ceramic rings are going on to two. When a tank is set up properly, these shouldn’t need to be replaced often, just an occasional rinse if things get sludgy.

What are your ammonia and nitrites? Because it’s possible an ammonia spike is responsible for the elevated nitrites – those nitrites have to come from somewhere. I’d perform back to back 50% water changes (separated across a few hours) to get your nitrites right down, 10 ppm is dangerous for your fish. I’d also check your nitrates, to make sure they are going up as usual. If they are remaining static, while your nitrites continue to rise, your cycle has crashed.


Awesome article. I have a QT tank set up which has been running for about 4 weeks. I added a bottle of ATM colony and added 1 fish. Its a 65 litre tank. After about 1 week, I tested my water last night and the ammonia has gone just 1 colour up from 0 (1.25) and same for the nitrite. I did a 15% water change as I thought taking any more out would mean taking loads of beneficial bacteria out as well?

I am also adding ATM first response whic is the same as Prime and detoxies Ammonia and Nitrite. My question is, shall I keep adding the fire response until the levels read 0? then keep monitoring my water to make sure that if those levels increase, I keep adding first response? and when finally the levels dont move from 0, It means the tank has cycled?

Many thanks in advance.

Hi Adam,

Don’t worry, beneficial bacteria are not free-floating. They cling to rough and porous surfaces and should be predominantly found in the biomedia inside your filter (such as ceramic rings) a water change won’t impact their growth.

Unfortunately, I cannot comment on ATM first response, I do not have any experience with the product. It sounds like you are attempting to do a fish-on cycle guide, I actually have a detailed step by step approach to this laid out here.

Have been cycling my tank for over 2 months. It’s a 20 gallon with 2 mollies in it. I currently have zero ammonia but my nitrites and nitrates are crazy high. Nitrites at about 5.0ppm and nitrates at about 80. I know I’m mid cycle and it needs to run it’s course but I feel bad for my fish!! I was very ignorant of the nitrogen cycle when I started my tank it else I would have done it fishless. I’ve been using prime and I’ve added aquarium salt. Is there anything else I can do to help them through it? Will 25% water changes harm my cycle?

Hi Karlie,

Water changes and prime are your best bet. Since the majority of beneficial bacteria will live in your biomedia in your filter, water changes won’t affect your cycle. Hang in there! It get’s easier after this part.

I made a huge mistake and listened to the pet store employee and brought my fish home the same day I bought my tank. It’s a 10 gallon. I have a Betta, 3 pearl danio and 2 snails.
Right now I have no ammonia, around 1ppm of Nirtite and between 10 and 20ppm of Nitrate. It’s been cycling since 3/24/19. I know now my ammonia is being processed, but my nitrites now are lingering.
I am doing water changes, dosing with Prime and Stability. I started out using Tetra Safe Start Plus, but I read that you aren’t suppose to do water changes while using that, and you can’t use Prime either. I didn’t like that, so I changed to Stability and Prime. Might take longer this way, but safer for my fish.
I have several live plants in my tank as well. I am hoping soon my cycle will be complete. Think I am on the right path?

Hi Sarah,

Don’t worry, we have all been there. You were lucky that you identified your tank needs to cycle, I have seen sooooo many beginners kill their first fish by following pet store instructions. It’s why I recommend independent fish stores over big box chains, the owners are in the hobby too and, generally give the correct advice.

As for your cycle, it sounds like it’s progressing just fine. If you started on the 24th, then it’s just going on a week, which is about the time you would expect nitrite eating bacteria to be growing in number. Exactly how much longer will depend, it’s unique to each tank, it may be a few days, it may be a few weeks. But it sounds like you are proceeding correctly you can check out my fish-in cycle guide for more info, but you are basically proceeding in the same way. With a little patience, you’ll get there!

Ian, help! Reading through these comments I’m afraid my cycle has crashed. Have had the 25 gallon tank since March 19th. Cycled only for two days before added fish. First 15 days seemed ok. But then had some fish die. Immediately did 50% water change and treatment. But for the last five days I’ve had nitrates at 40, nitrites at 10 ppm and ammonia at .5. These stats haven’t changed despite another water change of 15% and treatment of water. Had a fish disappear this am. After doing more reading…thinking I need to do daily water change of 25%. Confused about if I should Vaccum gravel or not. Also getting aquarium salt and biological media for my filter. What else should I do? I don’t want to lose any more fish…clearly they are stressed. Have really watched the feeding – do 2x a day and they eat within 45 seconds. Help!

Hi Margaret,

Two days is too short for a cycle to finish, it’s likely that over time the stress from ammonia and nitrite is responsible for the death of your fish. I highly recommend checking out my fish-in cycle guide to give yourself the best chance of saving the rest of your fish.

I’ve been reading everything I can on this subject. I’ve had my tank for about 3 weeks. I let it run with live plants for a week before adding my betta and two mystery snails. It is 5 gallon.

I feel terrible I didn’t know more about the cycle before introducing the creatures, especially mr. Betta. I’ve had a nitrite spike in the past several days, per the test strips I’ve been using. Looking back, I was overly enthusiastic To add a beautiful betta I had had my eye on for a week already and I really wanted to get him out of his tiny little plastic container at the pet store. I also think I was misinformed about mystery snails and wish I had just added one instead of two, mine are incredibly messy And have grown like weeds since I’ve gotten them. They even knock over decorations, approved plants, and fight with each other.

The problem is, I’ve been doing water changes daily like a fiend. I’ve been adding seachem stability, as I could not find seachem prime anywhere in my County. And yet, the nitrite levels Have still been testing daily in the high stress to low danger zone with seemingly very little change.

Do you have any ideas as to why these steps wouldn’t be seeming to make a difference? So far my livestock seems healthy and happy still but I’m so scared I am going to seriously injure or kill them with these water parameters.

Hi Em,

It sounds like you are doing the best that you can. Don’t feel bad, unfortunately, most new fish keepers learn about the cycle a little too late. If you can’t find seachem prime or another product to “neutralize” the nitrites then water changes with dechlorinated water are all you can really do to keep them as low as possible. Unfortunately, there isn’t a better solution here – if you can, perform them twice a day but once a day is still doing better than most. If you keep this up, you are giving Mr. Betta the best possible chance at pulling through. It won’t be forever, eventually your levels will stablize and then you can move on to weekly water changes.

I’ve been on your site many times in the past 5 weeks learning as much as I can, thank you!! I started with a small 5 gallon tank and added fish the next day, like the big box pet store told me to do. That tank actually did well and I didn’t lose any fish! But now I’ve upgraded to a 36 gallon tank and I know better. I started cycling the 36 gallon tank 5 weeks ago. I am following your instructions for cycling for beginners. It has been a test of patience, but I am hanging in there. I’m stuck on step 4 for a while now. I am getting 0ppm ammonia every 24 hours, my nitrites are holding pretty steady at 2.0ppm (the color doesn’t match perfectly and are very similar), my nitrates are 10ppm. I add a small dose of ammonia daily for the past 5 days and continue to get a 0ppm reading after 24 hours. I do have live plants in the tank, the filter has been running 5 weeks, we have the tank heated…fully set up except for fish. Do I just need to be patient and let the cycle continue so I can get my 0ppm reading on nitrites? Thank you again for this great website and your amazing knowledge!

Hi Cindy,

I know it’s painful testing each day, but it does get easier. It sounds like you are on the right path, you if the nitrite is being processed, you should see the nitrates rising. If you make sure you test under natural daylight as opposed to indoors, it will make it easier to match the colors (indoor lighting can throw the colors off) to me, it sounds like it’s just a waiting game for you.

Thanks for the input. I don’t mind the daily testing, I’m just ready to start reaping the rewards of my patience!

YAY! Ammonia and nitrites were zero today!!! But nitrates were high (40ppm), so I’m guessing that is the spike? I have added ammonia and will retest tomorrow. If ammonia and nitrite are zero again tomorrow I will add the full dose of ammonia and test one more time for zero readings. Should my nitrates level off by then or do I need to do a water change to lower those before adding fish? It’s getting exciting now!

Hi Cindy,

Congratulations, your hard work and patience paid off! I’m so happy for you.

In a cycled tank, with fish in it, nitrates will forever rise, its part of the reason why we perform weekly water changes, to lower them back down. Ideally you would want to do a water change or two to get these levels back down (under 10 is preferable in a non-planted tank) From here, if you don’t overfeed, maintain your tank and don’t overstock it, a weekly water change will keep nitrates at appropriate levels.

It’s almost fish time. You’ll feel like this was all worth it when they live long and full lives, without complications!

I’ve been keeping fish for many years now and have gradually progressed to marine. I now have a 1000ltr reef aquarium with a 200ltr sump. It’s been running and established for around 2 years now, I test the water every week. Recently I noticed I’ve been losing some of my crabs, mainly plating and acans. I tested for nitrites and was shocked to find them. They were around 0.5. I immediately did a 30% water change, tested again a week later, still the same so did a 50% water change. Again after a week they were still at 0.5. I did another 50% water change then added reef balance bacteria mix, 4 whole packs of them. A week later it was down to 0.25 and has stayed there for the past 6 weeks, I continue to do water changes every week (25%) change the filters every 3 days but cannot get rid of the nitrite. I’m now losing a well established torch coral and my green star polyp is starting to recede. Every other parameter is where it should be. Nitrates are around .2 and ammonia is at 0. Any suggestions?

Hi Clive,

Given the elaborate setup and that to keep it successfully run over 2 years, I am 100% confident you have checked off the most common causes.

My recommendation would be to ask this over at reef2reef perhaps someone there has experienced the same?

I’ve been trying to cycle a new tank for 4 weeks now and after having some guppies and ghost shrimp added to the water I’ve noticed the nitrites are starting to spike. Is the 30-50% water change a good idea in the case of a newly cycling tank? I’ve been doing 15% or so every 3 or 4 days. Will that set me back in the cycling process by removing so much good bacteria from the water since I clearly don’t have a filter currently capable of processing the waste. Thanks for the great resource.

Hi Blake,

Beneficial bacteria are clingers, they don’t float around your aquarium. They cling to things in your filter, like ceramic rings and sponge, where flowing water brings them food.

A water change is fully encouraged to stop these chemicals from harming your fish. Check out my Fish in cycle guide for more info, which covers everything you need to know in detail.

I’m at the 4-1/2 week mark of my cycle. Bought a new 55 gallon tank. Transferred all the water and gravel from my established 30 gallon tank with Fluval bio filter over. During the transfer something happened to my bacteria and the cycle crashed. Went to 8.0 ppm on the ammonia within a week! Started using API ammo-lock every 48 hours. I have 7 fish total, with the biggest being 3″ long. The other 6 are 1″ to 2″ long. Eight ghost shrimp, 2 small snails and some plants. Water temp is at 78 deg. One snail found dead this morning 9/4.

I bought a Fluval 306 canister filter and installed it on 8/3/19. I’m still running the original filter system running too.

Added a full 8.45 oz bottle of Tetra safe start plus. Still kept getting high ammonia. I perform 50% water changes faithfully. I’ve added 4 more full bottles of TST plus during this process.

Today (9/4/19) my ammonia is @ 1.0 ppm. but pH is dropping like a rock, from 7.4 down to 6.4 after a water change in 24 hours. Also my Nitrites are at 2.0 ppm after that 10% water change yesterday. I think I am getting close to the end of my cycling but do not want to injure my fish. Any ideas on what is going on and what actions I should take?
Thx, Lee

Hello I have also made a rookie mistake like some others in the comments above. I brought home a betta and set up his tank in the days that followed. It took me a while to find him the perfect tank, 6.5 gal with heater and sponge filter. I was clueless about nitrogen cycles until 2 weeks ago. My betta has lost his ability to swim correctly this has been about 2.5-3 weeks although I can recall some odd behaviors that I believe were symptoms just less severe. I can elaborate if you need me to.
I haven’t seen any sign of bacterial or fungal infection (although before he fell ill I used melafix to hell heal a fin he had ripped) now realizing that wasn’t the best idea.

He mostly sits on his side at the bottom of his tank and it takes a lot of effort for him to flip over or move around to get food. He still eats enthusiastically and I’ve been careful to not over feed him or cause constipation. Currently I have him in a nursery with some plants to be closer to the surface of the tank although he hasn’t managed to mke it up for air in a few weeks. I lift his nursery a few times a day for him to get some breaths which he happily accepts.

I’ve had him since the beginning of July and I believe I was unknowingly collapsing the nitrogen cycle by very thoroughly rinsing his filter in tap water every week with a water change. His ammonia will spike to .25 or .5 and I will do a water change, then a nitrite spike will occur to similar levels and I will also do a partial water change but his nitrates remain very low. Between 0 & .5 and that’s a generous estimate.

He seems to be showing some progress now that I’m aware of the nitrogen cycle, but only on the way that he seems alert and like he’s trying to move around more. This is mostly unsuccessful but we have some small victories like a big leap or a flip or balancing on his belly instead of his side.
but how do I get the nitrite eating bacteria up so his tank won’t need as many water changes?
And if you know anything about this , can ammonia or nitrite spike have this effect (causing fish to be unable to swim)?

Other information he had small and large gravel, some Marimo balls and java ferns in the tank with him.
I feed him frozen brine shrimp, frozen blood worms and recently introduced midge larva pellets. He once got live mosquito larvae, that was fun for both of us! but that was before he fell ill.
Tank temp is usually around 80 give or take a few degrees depending on the weather that day (even though it’s a heated tank).

Hi Katrina,

Don’t feel bad, this is unfortunately a right of passage for many beginners into the hobby – this cycle isn’t made apparent enough. I must congratulate you for taking the initiative to do your own research and take steps to make it right!

Poor water quality is most likely the cause of your bettas problems. Ammonia and nitrite spikes can absolutely cause fish to be lethargic. Unfortunately, it *may* be too late and the damage is done. unfortunately, fish can hold on for a long time before giving up the ghost. This makes it difficult to identify the true cause of their death – it may be from something that occurred months ago.

The only option here is to move forward. Bottled bacteria, such as tetra safe start *can* help the cycle progress quicker in some cases and may be worth a shot. Otherwise, if you are doing everything else right, you may just have to wait it out. Much of the cycling process comes down to waiting and testing.

I have a guide on performing a fish-in cycle too which may help you:

Thank you so much!! I’ll look into tetra safe start and check out your guide now!

Apart from being a now crooked and lethargic swimmer he seems to be normal in all other ways. Appetite is still very strong and he does recognize and enjoy seeing me when I check on him multiple times a day.
Now, If I can only get him back to swimming normal. Fingers crossed with proper tank maintenance that my guy miraculously makes a comeback !

I hope it all works out. Please post a comment on the cycling guide if you have any other questions about the fish-in cycling process!

Hello! I thought I’d reach out and update you on his condition/progress. After introducing the tetra safe start bacteria to his tank I saw an immediate change in water conditions. Finally some nitrite to nitrate conversion! The nitrite level has been 0ppm ever since, nitrate around 5-10ppm. Ammonia hovers at .25 a .5 ppm, I’m still doing small water changes every other day to keep those from spiking, but they haven’t gone above .5ppm

He is now out of his floating nursery because he can swim again! After having much trouble getting off his side for the past 3 weeks! He’s now enjoying a slightly lower water level with a bunch of new plants to swim in/sit on. Thank you for your help! Im so happy to see him playing and exploring the way he did when I first got him.

He is still a bit tail heavy, but I’m not sure if that’s a damaged swim bladder condition or a product of over breading, his fins are quite large for his size.

Thank you again:)

Hi again Katrina,

Oh, I’m so happy to hear that progress is being made. Thanks so much for sharing the update. It sounds like you are all over the routine with getting ammonia down and hopefully the ammonia goes away soon too – it will be one less thing for you to worry about!

It will be interesting to see if his condition improves over time once the water quality is perfect. I hope he does.

Thanks again for the update!

Hi Ian, really hoping you can help, as we seem to repeatedly get bad advice every time we ask at the local fish store/aquarium. Long story short, we had one betta in a 10 gallon tank for a few weeks. Water testing was ok but we had a major problem with algae that we couldn’t keep up with through cleaning so we took the advice at the pet store to get “a few” snails and ended up (on their advice) to get 5! Within a couple of days the algae was completely gone, but there was SO MUCH snail poop in the tank (which of course makes sense!) and the ammonia, nitrite, and nitrate levels were all through the roof! We have re-homed 3 out of 5 of the snails, have vacuumed the substrate every 1-2 days and done partial water changes for the past week and are now at the point that not much poop comes up, but all of the ammonia/nitrite/nitrate levels are still elevated. Yesterday I did an 80% water change but still when I checked this morning, all levels were elevated. We have been using Prime water conditioner with the water changes to neutralize the harmful effects of the water being off – is there anything else we should be doing?? Thank you!

Hi Rachel,

When you say elevated, what are the exact readings for:

Water temeprature

Was your tank cycled before you added the snails? Or has the cycle crashed as per the test kit results? It’s possible that you need to cycle the tank.

Here is a guide on fish-in cycling.

Otherwise, it sounds like you are on top of keeping your fish safe – I must commend you for that. Just be mindful that 1 “dose” of prime treats 1ppm of ammonia (or ammonia + nitrite added together) if you have 2ppm, you’ll need a double dose and so on.

Thank you for your reply! It is hard to say exact readings since our kit (the API master test kit) gives readings in increments, but here are approximate readings as of today (after doing another 50% water change yesterday):

PH: 7.6 (the colour seemed about half way between the 7.4 and 7.8 shades given)

Ammonia: 0.25 ppm (seems pretty bang on the shade given for this reading)

Nitrite: 0.5 ppm (maybe a bit lower – definitely darker than the 0.25 shade but perhaps not quite as dark as the 0.5 shade)

Nitrate: 5.0 ppm (perhaps a tiny bit darker but not as dark as the 10 ppm shade)

I also tested our tap water for ammonia and it read 0.5 ppm! That explains why we always get a slight ammonia reading, but before the debacle with the snails the nitrite and nitrate always read zero.

Water temp fluctuates between 78 and 80 F

We did not properly cycle this tank before adding adding our betta – we used a quick start product (nutra fin).

What is our best course of action here?


Hi Rachel,

Even if your tap was has 0.5 ammonia, this will soon drop to zero in a cycled tank. You would expect that if you tested the tank 12 hours to a day after your water change, the ammonia would reach zero.

The presence of nitrite confirms that the tank is uncycled.

Based on what you have said, it’s unlikely the snails were the cause. Rather, nitrite and nitrate will always read zero in a completely uncycled tank (You would expect some level of nitrate to always be readable) The appearance of nitrite and nitrate coincided with the snails and they were the blame.

“Quick start” products do not instantly cycle the tank. This advice is often repeated by poorly trained petco or petsmart employees who do not know any better. Don’t worry, you are not alone here, most beginners experience this bad advice.

You’ll need to do a fish-in cycle. Check out the link in the previous reply, it will get you back on track.

Thanks! I’ve read your article about fish-in cycling. Just a few questions/points of clarification. Since there is ammonia, nitrites and nitrates all showing up, I’m assuming I am going to start at step 6 of the process (even though ammonia levels are not yet zero, as referenced in the instructions). And just to confirm – I do not factor in the nitRATE reading when determining when to do a water change or when dosing with Prime? For example with the readings I gave you above, I would not do a water change since ammonia and nitRITE are below 4ppm combined, and I would use a single dose of Prime since they are below 1ppm? How do we protect against the high level of nitrATES at this stage?

Also, on a day that do a 50% water change, do I dose based on the water replaced or the entire volume of the tank? Thanks so much for your help!

Your thoughts are entirely correct. Nitrate is excluded from the calculation. In small doses (typically under 40ppm) it’s harmless to fish. Any water change you do to get your ammonia and nitrite down will also drop the nitrate. In most cases, you shouldn’t need to worry about it until the end of the cycle. If nitrates do explode, water change as usual.

Remember, a 50% water change will roughly drop ammonia, nitrite and nitrate by 50%. If your levels are exceptionally high, you may need to do multiple changes. For example, 160 nitrate at 50% only gets you down to 80.

During the cycling process, dose for water replaced then dose as usual. It’s only a single dose. Prime can be dosed for up to 5x safely. Since we are capping out dosing at 4x (combined ammonia and nitrite) you should never exceed 5x, especially with the 50% water change dropping these levels in half.

Please let me know if anything I have said here needs clarification!

Hi Ian, didn’t look like there was a “reply” button under your last comment, so I apologize for getting things out of order a bit by replying higher up. I want to make sure I understand the Prime dosing correctly using a couple examples.

Example 1, no water change: I test and get readings of 0.5 for ammonia and 1.5 for nitrite. I do not do a water change because levels are lower than 4ppm. I dose with Prime based on the total volume of the tank (10 gallons) and add 2 ml of Prime to the tank (2x the single dose for 10 gallons, which is 1 ml per the bottle). Is this correct?

Example 2, 50% water change: I test and get a reading of combined ammonia/nitrite reading of 5ppm, so I do a 50% water change. I first divide the ppm in half because of the 50% water change and I am left with 2.5ppm, so I add 3ml of Prime to the tank with the new water. Is this correct?

I also have a question about the filter we are using, based on reading comments from others, and your replies. We are currently using the Marina slim S15 power filter, which has slots for 3 cartridges. We change one out of three cartridges every week, per the filter’s instruction manual, and currently have two “bio-clear” cartridges, which combines Zeolite and Ceramitek, and one “bio-carb” cartridge, which combines combines activated carbon and Ceramitek. Are we slowing down the cycling process by using the cartridges with zeolite and should we replace them with the bio-carb cartridges?

Thanks again!

Hi Rachel,

Your examples are spot on. Going one one step further, I would advise that you test your tap water before adding it. Some (but not many) locations in America can have ammonia or nitrite in the tapwater, so you would need to account for this too.

Any filter where you throw it out weekly or monthly is not just costing you money, but slowing down the cycle. They are a bit of a scam and I wish that beginners would stop buying them so they would be gone from the hobby for good.

I am unfamiliar with the Marina Slim S15, but if you can fit aquarium sponge (buy it in sheets and cut it to size) and ceramic noodles, this will be all you need. Sponge lasts a year or two, and ceramic rings can last up to 5 or more depending on the type.

Rather than dispose of them, you would rinse the sponge in the water you removed out of your tank every few months during the water change. The ceramic rings hold most of the beneficial bacteria, and these can be left untouched for the most part, since the sponge stops the gunk from reaching them. You’ll know they need to be replaced when they start to dissolve or become smooth (the beneficial bacteria lives in the tiny holes)

Let me know if you need any extra info!

Oh yes, of course – as I mentioned in an earlier comment, we do have 0.5 ppm of ammonia in our tap water, and now my head is officially spinning with the math! I feel awful that I’m doing water changes and adding more ammonia than I am taking out! But alas, I will continue and hopefully things settle in soon (it’s a good sign that I’m seeing nitrate, right? Means we’re at the last stage of the cycle?). Thanks again!

Sorry Rachel, I must have missed that in the earlier comment.

Don’t feel bad about adding the ammonia! This is all part of the process of fish-in cycling. Your math was spot on before and I’m sure this tiny extra part of the equation won’t cause you any grief. Everything sounds positive and it looks like you have a great eye for detail. It’s a very good sign that you are seeing nitrate, it means the cycle is nearing it’s end. Of course, it could still be a while, but as long as your nitrates keep going up (excluding your water changes) then everything should be progressing as intended. Be patient, it will all be worth it in the long run!

Hi Ian, great site been really useful cycling my first tropical tank.

Just after a bit of advice please. I used fish in cycle with two danios in 120L planted tank. On day five Nitrites started to increase steadily, over the next 10 days peaking to 1.0ppm, nitrites were then starting to drop over the next five days to 0ppm so all good. Nitrates started to rise from day 9 and have been controlled with partial water changes.

So by 20 days, tank appeared to be cycled, waited another 5 days with 0ppm Nitrites then added 10 shrimp and 5 snails. (Note, around 5 days before i added the shrimp i was getting a little bit of algae build up on rocks and structure and plants, within 1 day of shrimp and snails going in 95% of the algae had gone! including on the plant leaves)

Two days later the Nitrites had risen to 0.25ppm did a water change of 25% and dropped them down a bit, now back up to 0.25ppm over a couple of days but holding steady now.Nitrates between 20-30ppm

Surprised the shrimp and snails may have caused this, is it a mini cycle? Although shrimps seem to poo allot! If Nitrite levels do not go higher than 0.25ppm should i just wait for the tank to sort itself or could i kill the shrimp over time at 0.25ppm? (They are still feeding happily)Or should i do a few partial water changes to keep Nitrites lower than 0.25ppm? Any advice would be welcome.

Fascinating hobby 🙂



Hi Stuart,

Welcome to the hobby!

It sounds like you have your head around the cycling process. I suspect, based on what you have said, that it comes down to how you cycled your tank.

You see, the beneficial bacteria establishes itself to equalize the amount of waste being produced. The amount of beneficial bacteria in the tank can only eat as much as the two danios produced.

This is part of the reason why I don’t like fish-in cycles and prefer the fishless method, where you dose more ammonia than your fish would produce and the beneficial bacteria “dies back” to match the amount of ammonia your newly added fish produce.

What I *believe* happened here is that when you added your shrimp and snails, you significantly increased the amount of ammonia that was being produced. The beneficial bacteria now has to grow to accommodate this extra waste. So, in a sense, your cycle is still not complete.

Fortunately, with an already established beneficial bacteria colony, it shouldn’t take too much time for the cycle to complete this time. As for the damaging effects, it’s hard to say. Some shrimp are really sensitive, others will survive through low levels like this. I’m of the opinion that partial water changes are always a good thing in fish-in cycling, as long as you are up for the extra work.

I hope this helps!

It sure is a fascinating hobby. It’s as much chemistry as it is raising a pet.

Hi. I did/do have an established tank I’m not sure anymore. Currently have 3 axolotls in a tank and do weekly water changes . I use prime and stability and also have matrix in my filter. I’m not sure what has happened but currently my water test is showing signs as if it is cycling. My ammonia is quite dark green but my ph is low making it less toxic but this week I have signs of nitrite. They are eating as normal but have started to reduce how often they have food in case their waste is the issue and my fluval u4 may not be working well enough. I also have air stone in tank as well. I do use sea hem ammonia alert to let me know how much of that ammonia is extremely toxic. Is my tank recycling and has crashed plz. I did have plants but they destroyed them so I’m curious into thinking if the left over peace’s that I keep finding is contributing to the readings and once I can remove every dead unseen plants will then things recover. No idea but making up possibilities as I go along

Hi Lauren,

Unfortunately, I have no experience with axolotls. So anything that follows is advice around fish…

If you are getting signs of ammonia, then it’s likely that your tank has crashed and you’ll need to cycle the tank from the beginning. In a fish tank you would do a fish-in cycle.

Instructions here:

Great info, thanks for this article. If you notice your fish has nitrite poisoning (bending tail), is it possible to help that fish or is it permanently like that now?

Thank you for your comment! Symptoms like a bending tail can indicate serious stress or damage, but many fish can recover with prompt and proper care. Improve their environment, maintain good water quality, and keep stress to a minimum. Monitor your fish closely and consider consulting a vet if you don’t see improvement. Recovery is possible with timely intervention!

Leave a Reply

Your email address will not be published. Required fields are marked *