Last update: November 12, 2022

Aquarium Nitrates and how to get rid of them!

You might not be able to see them, but nitrates are hiding in every aquarium.

Understanding this invisible compound is an important part of maintaining happy and healthy fish.

Today, I’m going to teach you everything you would want to know about them.

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

This cycle forms the foundations of what I am about to tell you.

Got it under wraps?


Then, let’s continue….

What are nitrates and where do they come from?

Nitrates (NO3) are a form of dissolved nitrogen that occur naturally in the water column , which is the water inside your aquarium.

Exactly how nitrates get into your aquarium is quite a journey… Nitrates are actually a by-product of biological waste.

Common sources of excess waste found in aquariums include:

  • Uneaten fish food
  • Fish poop
  • Decaying plants
  • Dirty filters
FishLab Fact: Overfeeding is the number one cause of high nitrate levels in aquariums. Not only do overfed fish poop more, but any uneaten food rots, leading to even more nitrates. Overstocking your aquarium comes in as a close second – watch that bioload!

But while waste results in nitrates, it is not the cause of nitrates.

To explain, I am going to do a very brief recap of the nitrogen cycle.

Before waste becomes nitrates, there are a couple of steps that happen first.

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 without nitrites, nitrates in your aquarium wouldn’t exist.

Nitrite converted to nitrate with bacteria in aquarium diagram

I know there is only a single letter separating the two – nitrIte versus nitrAte…

But don’t get confused. The two are very different.

Nitrites are highly toxic to your fish, while nitrates are considered harmless, at least in small amounts.

Now, I must stress this… harmless in small amounts.

Over time, nitrates will build up in your aquarium. This process is continuous and unavoidable.

When this happens, you need to reduce the nitrate levels in your tank, which I tell you how to do later in this guide.

What happens if your nitrate levels are too high?

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

I’ll be frank with you, unless you stock sensitive fish or corals, it’s unlikely that an increase in nitrate levels is going to outright kill your fish.

But that doesn’t mean that high nitrate levels are not dangerous in their own way.

You see, high levels of nitrates place your fish under incredible stress.

And when your fish are stressed, the following problems can arise…

1. Increased chance of disease

Have you ever gotten so stressed that you became sick?

Well, that’s similar to what happens to your fish. The stress caused from increased nitrate levels make your fish more susceptible to diseases.

2. Poor growth

Fish that are exposed to high levels of nitrates may have stunted growth, meaning they may never reach their full size.

3. Poor color

Fish that are stressed by high levels of nitrates won’t display their natural coloring the way they would under normal, healthy conditions. You may even find that your fish’s color will fade.

4. Damaged reproductive organs

If you are trying to breed fish, then you definitely want to keep nitrate levels low! High levels can damage reproductive organs, meaning your fish will be unable to make babies!

5. Reduced life span

Fish that constantly live in water with high levels of nitrates do not live as long.

While some of these problems are reversible, others cause permanent damage to your fish or even shorten their lifespan.

To put it simply, if you want to raise happy and healthy fish, you need to get your tank’s nitrate levels under control.

What are the right nitrate levels for your aquarium?

The ideal nitrate levels for your aquarium depend on two factors:

  • The type of tank (freshwater, saltwater, etc.), and
  • The type of fish, plants or invertebrates you stock.
FishLab Fact: The measure for nitrates is ppm (parts per million). For you metric folk, this is the same as mg/L (milligrams per liter).

In a perfect world, nitrates should be kept between 5-10 ppm or lower.

However, this is not always the case…

Whether it’s due to a lack of time to perform proper maintenance, laziness or just a lack of understanding, fish often live in extreme nitrate levels.

But just because your fish are surviving, that doesn’t mean they are happy or healthy.

Below are suggested upper limits for nitrate levels that should not be exceed for freshwater and saltwater tanks.

Freshwater tank nitrate levels

Freshwater tank< 40 ppm
Planted tank< 30 ppm
Brackish> 50 ppm
Pond> 50 ppm

Saltwater tank nitrate levels

Reef tank< 1 ppm
FOWLR< 30 ppm
Coral tank0.25 ppm

Note: The nitrate levels above are general guidelines. Individual species of fish, plants or invertebrates may have more specific requirements. And remember: Just because your tank is within those limits doesn’t mean that your fish are happy.

As you can see from the charts, marine fish are much less tolerant of high nitrate levels than freshwater fish.

FishLab Fact: High nitrate levels can lead to disease, cause reproductive failure and may even stunt growth in fish.

If you keep a hospital tank for any sick or injured fish, keep the nitrate levels in this tank below 10 ppm.

It is also worth mentioning that many varieties of algae feed off nitrates in the water.

Many algaes use nitrates as a food source. In this case, I recommend keeping the nitrate levels below 10 ppm. This helps deprive the algae of its food source, making it easier to eradicate.

FishLab Fact: The “lower nitrates are better” rule is thrown out the window if you have a planted aquarium. Many plants use nitrates for food, and in this case, low nitrate levels can actually cause problems and can even lead to a blue-green algae (cyanobacteria) outbreak.

How do you test for nitrates?

To identify the nitrate levels in your aquarium, all you need to do is go out and buy a colorimetric test kit that is specifically for nitrates.

Nitrate test kits are typically designed to work for both saltwater and freshwater aquariums.

If you have ever tested pH , then you will know how these test kits work.

Take a sample of water from your aquarium, add a couple of drops of the solution included in the test kit and watch the water change color.

Below are two of the best nitrate test kits around:

API Nitrate Test Kit

Api Nitrate freshwater and saltwater test kit

Lasts up to 90 tests If you walk into your local fish store, this will likely be the test kit they keep on the shelf.

What can I say? It’s cheap, and it works. For the everyday fish keeper, the API test kit will be more than appropriate.

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

However, since nitrate test kits are technically measuring nitrites, they have the same shortcomings that the nitrite test kits have – amine interference.

In certain cases, such as in reef tanks, essential amino acids – amines – can cause the tests to show a lower nitrate level than what is actually occurring in your tank.

Fortunately, there are test kits that get around this issue.

Unfortunately, many of these kits are expensive.

But not this one…

Salifert Nitrate NO3 Test Kit

Salifert nitrate test kit for aquariums

Lasts up to 60 tests

This nitrate 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.

The Salifert nitrate test kit is popular with many saltwater aquariums owners.

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

While nitrate 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.

How do you remove nitrates from your aquarium?

Just tested your nitrate levels and discovered it’s through the roof?

Don’t panic!

I am now going to cover all the ways you can lower the nitrate levels in your aquarium.

But before we jump into this section, I want to make one thing clear:

The battle against nitrates is never over!

As long as your fish are eating and pooping, there is going to be waste in your aquarium, which means that unless you act, your nitrate levels are going to continually rise.

The only way to win this battle is to remove every fish, Aquatic plant and natural organism from your tank – at which point, I would hardly call it an aquarium.

If that isn’t an option for you, and it shouldn’t be, then you need to monitor and reduce the nitrate levels on a regular basis.

So, the key takeaway is:

You cannot eliminate nitrates – you can only control them.

Now that we have that out of the way, let’s take a closer look at the different ways you can remove nitrates from your aquarium.

1. Water change!

A partial water change should already be a part your maintenance routine.

And if it isn’t? Well, it’s no wonder your nitrate levels are so high.

A water change is a great way to instantly lower the nitrate levels in your aquarium.

You should also be mindful that nitrates can be present in tap water. This is a particular problem for those who draw water from wells.

If you use tap water for your water change, test for nitrates before putting it in your tank. If high levels of nitrates are found, consider using a different water source, like reverse osmosis (RO) water.

If your aquarium is under control, a regular water change should be all that is needed to adequately reduce nitrates to safe levels.

However, for those who need a bit more help in the fight against nitrates, there are three more solutions to lower the nitrate levels in your aquarium.

Be mindful that these extra tips for removing nitrates won’t replace a water change – they should be used in addition to it.

2. Water conditioner

This is essentially a nitrate remover in a bottle. Simply add the recommended amount and watch your nitrate levels drop as if by magic.

Exactly how the water conditioner reduces nitrates varies from brand to brand. Some convert the nitrates to nitrogen gas, allowing it to exit through the surface.

Other water conditioners bind the nitrates, rendering them harmless to fish and making it possible for the bacteria in your biological filter to destroy them.

Whatever the method, the end result is the same – lower nitrates.

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

Seachem prime best concentrated water conditioner for marine and freshwater tanks

Not only can it detoxify nitrates, but it can also deal with nearly any other water quality issue that may arise!

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

3. Nitrate removing filter media

While the bacteria in your filter might do a great job of getting rid of nitrites, nitrates are another story.

You can give your filter a turbo charge by using a media that traps nitrates, removing them from the water.

You can also use a bio-media, a media that uses bacteria to filter the water, for nitrate removal…

Siporax glass bio filter media for nitrate removal

If you have been in the fish-keeping game for a long time, you might remember the name Siporax. If you are like me, you probably thought that it had vanished forever.

But it turns out that Siporax is fantastic at encouraging the growth of denitrifying bacteria that breaks down nitrates!

Realizing its potential, Siporax was reintroduced to the market by Paul Hughes, who owns one of the most impressive display tanks in the entire United Kingdom. [11]

There are also nitrate reducing pads…

Acurel nitrate reducing filter media pad

There are also nitrate reducing pads… These pads are infused with nitrate-reducing agents to trap and remove nitrates from your water.

The best part about these nitrate pads is that they can be cut to size.

It doesn’t matter whether you have a small HOB filter or large pond filter, you are guaranteed that the pad will fit.

However, nitrate pads do have an obvious drawback – they need to be replaced constantly. The typical lifespan for a nitrate pad is 2-3 months, after which it needs to be tossed in the trash.

4. Get planting!

Plants naturally utilize nitrates.

This is why high nitrate levels often result in an algae outbreak.

However, it is important to note that plants draw nitrates out of the tank water at different rates.

In fact, most plants are rather ineffective at lowering nitrate levels.

However, there is one planting solution that was designed specifically for nitrates… Rather than planting inside your aquarium, consider planting inside your filter.

Plants such as lucky bamboo and pathos not only look great when growing out the top of your aquarium, but their roots draw up nitrates as they pass through your filter, removing them from the system.


While there is no arguing that the methods listed above work exceptionally well at reducing nitrate levels, there is one thing you need to be aware of:

These methods only remove nitrates – they don’t fix the cause of them.

If your nitrate levels are constantly spiking, then it won’t matter how often you reduce the nitrate levels in your aquarium. The cause is still there.

But don’t worry, I have a solution for you…

How do you prevent nitrates from building up?

If you read my guides on ammonia and Nitrites, then you will already be fully aware of the advice I am going to give you.

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

Waste -> Ammonia -> Nitrite -> Nitrate.

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 nitrates in your aquarium, they will stop new nitrates 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 nitrates will build up inside your aquarium.

I would also add that any food that remains uneaten sinks to the bottom of your aquarium and rots. And, you know what that means? More 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 want to incorporate a bit of underwater gardening.

Over time, old leaves on your plant will die.

When this happens, the leaves fall off and decay, which leads to an increase in nitrate levels.

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 organic matter and poop? Well, they are sucked up and trapped by your filter.

With time, this waste accumulates. When this happens, your filter turns into a nitrate factory.

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 aquarium water, trapping it before it reaches your filter.

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

4. Grab a gravel vac

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 nitrates 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 be performing a gravel vac during most, if not all, water changes.

5. Monitor your stock

An overstocked aquarium is going to have more nitrate problems than an understocked one.

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

If your fish 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.

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 nitrate 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.


Phew! If you are still with me, then you made it to the end of this guide!

I know it’s a lot to take in, but nitrates are an important part of every aquarium, and understanding their role and how to control them is essential to raising happy and healthy fish.

Do you have any nitrate tips? Let me know in the comments below!

Ian Sterling

I've been keeping fish for over 30 years and currently have 4 different aquariums – it's an addiction. I'm here to teach you everything there is to know about fishkeeping.

I also use this site as an excuse to spend lots of money on testing and reviewing different aquarium products! You can find my reviews here.

Comments (67)

I have 40 koi. In 18,000 pond.the filtration up the yingyang the turnover rate is 1-1/2 times per hr.
I clean the filters weekly. But only about 1 thousand gallons. Water temp was 88 degrees Fahrenheit. For 2 weeks I try and keep it shaded all day. But it wasn’t working so I did a major water change 18 hrs.the nitrates went from 20-ppm to 0-ppm. And I have salt level @ 3%. Some fish were showing signs of stress so that’s why there’s 3% salt. Also had a huge algae bloom. How long should I leave the salt level there.
Ten years and never had a problem. I live in Chicago and I heat the pond all winter at 72 degrees until spring time. And never ever tested the water… so ok the fish are growing more I’m on top of the filters but not enough water changes. ???? Thanks Arthur.

Hi Arthur,

Unfortunately, I do not have any experience with ponds – our main focus at the moment is building the best online aquarium resource that we can.

While we may expand to ponds in the future, we currently do not have any staff experienced in owning and maintaining ponds.

Hi Peter,

Thanks for sharing!

Sounds like a custom filter designed to help denitryifying bacteria to to grow, these bacteria feed on nitrogen. These are the same anaerobic bacteria that I mentioned earlier in this guide, although this filter takes that concept and significantly ramps it up.

Denitrifying bacteria converts NO3 to N2 in anoxic conditions which is slightly different than anaerobic conditions.
Dissolved oxygen levels in anaerobic conditions is absolute 0 mg/l, in anoxic conditions it is between 0 and 1 mg/l but not 0.

I have big nitrate problem I’ve did many water change and still have the problem you have any other suggestions I’ve vacuum the bottom 2 or times changed all filters ?

Hi Joe,

That’s no good. If nitrates are accumulating to dangerous levels before it comes time to your weekly water change then it’s possible that your tank is either overstocked or you are overfeeding your fish.

I have the same issue did everything possible and nothing worked. I then decided to buy an undergravel filter and 2 power heads and with in 3 day my nitrates are at zero.

Hi Shawn,

While I thank you for weighing in, nothing that you have listed here will reduce nitrates to zero.

Aloha. I have a 5 gal freshwater tank with a single betta in it and about 6 live plants. I tried to look for info about where I might be in the nitrogen cycle– I tested my water this morning and have no nitrates, but a small amount of nitrites. Could you perhaps provide some insight as to where you think I might be? I think I had a small algae bloom last week, but now the tank is super clear (well, as clear as a blackwater tank can be, lol…) Mahalo.

Aloha Kikilia,

Nitrites are the second stage of the nitrogen cycle. These will continue to build up until bacteria establishes itself that converts the nitrites into nitrates. As for how long this takes? It depends. For some it’s a few days, others it’s a couple of weeks.

If the algae bloom turned your tank foggy then it was likely a bacterial bloom, as discussed in our cloudy aquarium water guide. This harmless cloud commonly appears as your tank cycles and quickly disappears.

Here is a chart I created the last time I cycled my tank, to keep track of it. You can see that as ammonia levels drop to zero, nitrites appear – this is the second stage.

Hope that helps!

you say high nitrates creates algae but that algae is also a plant which consumes those nitrates isn’t it? Actually i believe nature automatically creates those algae to fix nitrates issue just like it creates beneficial bacteria when there are Ammonia and nitrites…

I have a mini pond openly placed in sunlight with highly stocked fishes and algae growing on sides and rocks and i have zero nitrates… Whereas i have another tank inside my house with medium stock and no algae and my nitrates stay always 40ppm no matter how clean i keep… So i believe the water with algae is not actually dirty but pure, Bcoz it sucks up all nitrates…

Hi Karthic,

Thanks for your insightful comment.

You are spot on, in an aquarium setting, algae often appears as a result of a nutrient imbalance, often an excess of something, such as nitrates.

Because the goal of aquariums is often to keep the water as clear as possible (it makes it easier to view fish) algae is unwanted, no matter how natural it may be. It’s interesting isn’t it, how often do you see perfectly clear water in nature?

Outside, such as your pond, there are plants, algae and even bacteria that eat nitrates. The ecosystem works as a whole to balance out (things wash into the pond, etc.) because the aquarium is a closed system, we have to create this balance ourselves.

Hello my problem is that my nitrates are at 150 I clean the tank every week and I had my fish for 3 years I have a Oscar my ph is 72 I clean the tank today and still at 150 what’s up with that I need some help.

Hi Robert,

Oscars are messy, it’s possible you are not performing water changes frequently enough to keep up with the waste produced. This is a common problem when a tank is too small for a particular type of fish.

Hi I have a 75 gal saltwater aquarium and 25 gal sump now I have a green hair algae all over on my rock I try to clean but it still come back I do water change weekly 15 % I use RODI water I test the water and my PO4 is 0.03 and my NO3 is about 50 to 80 so I went to the local store and they told me to use nopox I just ordered I need help please

Hi Shurhrat,

80ppm of nitrate is quite high for a saltwater tank, especially if you have corals. It’s possible these high levels of nitrate are responsible for your algae outbreak. You’ll either need to make a change to your tank or routine to lower these levels. Once you have the nitrates in check, manual removal of the algae should be enough to prevent if from coming back.

Hello Ian…

Have been in the water keeping hobby for a number of years and everything I’ve read to date has said there isn’t a bacteria that will use nitrate. This fact requires that we perform large, frequent water changes to remove this form of nitrogen from the fish tank. Lately, I’ve heard that such a bacteria does exists. Is this true?

Hi B Bradbury,

Yes, this is true. It’s an anaerobic bacteria that lives deep in media without air. In a freshwater tank, it’s not going to remove enough to offset water changes due to the amount of media that would be required. Besides, a water change has other benefits such as restoring essential nutrients. However, their are certain medias that encourage this bacteria to grow. In a saltwater tank, live rock carries these bacteria.

I’ve just read your articles on Nitrates. I have a freshwater aquarium with fancy goldfish and have been using the JBL Pro Scan strip tests which I use at least weekly. I need advice as the NO3 has been stuck on 50 for 4 weeks, despite changing 60% of the water each week. The strips are in date, and I also test at the same time per day with the test card in the same position, under natural light. I performed the test immediately before the last water change (c.60%) and immediately after it (45 mins later) and the reading was the same !…… My fish seem happy enough but I’m guessing I need to switch to a liquid reagent test kit !……. Would this be your advice ?

Hi Gary,

I personally don’t recommend test strips because they often return an incorrect reading, even when it looks like everything in order. Test kits are more affordable (the API master test kit has everything you need for hundreds of tests) and much more accurate. Once you have double checked your results, then you’ll be able to take appropriate action.

I find the strips are NOT useful. I brought a few to my job (a lab) and it is not the specificity, but the sensitivity I had a problem with. My (very crude and casual) experiment explained why my strips tested safe levels but my fish were progressively getting worse with fin rot. With the wet chemistries in the API, I get – IMO- the sensitivity needed to catch problems sooner. I am cycling a new 20g H tank right now. JUST. started catching a 0.25 AMMo. I dipped one of the last sticks I had left over and it was negative. Now I am crossing my fingers for my betta and platys. Thanks for a simple, easy explanation.

Hi Mary,

Thanks for sharing your experience on strips vs. liquid test kits. You are 100% right, being able to catch ammonia just as it appears is going to save your fish a lot of suffering compared to a strip which may catch it at a higher level.

I have a 55 US gal tank and my filter is a “gravel filter” with two PH-16 Aquatop pumps in each back corner of the tank, circulating the water through the gravel. I know most Pros think “gravel filters” are “old school”, but works for me.

My question is: Could I use the Sera Siporax product and sprinkle on top of the substrate (gravel)?

Hi Dave,

Yes you could. Just be mindful that over time your under-gravel filter will clog and the water flow will reduce. This will reduce the efficiency of it to filter your tank. When this happens, you’ll have to do a deep clean which will involve removing your substrate. It’s for this reason I recommend other types of filter that sit inside or outside your tank instead.

Are there any types of Nitrate reducers that can give a false positive? I have a 120 gal and a 75 gal salt water tank. I perform water changes bi-weekly and have in my large tank, in my sump, a good protein skimmer, a nitrate pad, and I use a natural Nitrate Reducer (Instant Ocean brand). My levels are between 120-140 ppm. My 75 gal, I have a good HOB protein skimmer and a reactor with NP active Pearls. My 75 gal levels are lower, but it is still about 60 -80 ppm. I do not over feed and my bio-load is 8 fish, about 15 hermits and 1 emerald crab in the 120 and 6 fish and about 20 Astraea Turbo’s in the 75. I was told that the nitrate reducer can give you a false reading. I want to put a bubble tip anemone in each tank but I know they are susceptible to nitrates. Any help would be greatly appreciated.

Hi David,

Unfortunately, I have don’t have a whole lot of experience pertaining to use of nitrate removers in saltwater tanks. This is a question you should ask over at reef2reef or reefcentral, these are great forums and I am confident someone over there will be familiar with your problem. I wish I could help you further.

Being an acid (nitric acid) isn’t it possible for high nitrates to kill beneficial bacteria and bring about a total tank crash (i.e. the conditions of an uncycled tank – a deadly ammonia spike)?

Hi Ben,

Absolutely, but this would be an extreme scenario. In all my years, I have never heard of this occurring since water changes are used to maintain the nitrate levels. If your nitrates get so high that they are rapidly dropping your pH then it would be due to poor practices such as not maintaining your aquarium or overstocking your tank to ridiculous amounts. At this stage, your dead fish would likely be the clue that something is wrong.

great info ! been into fish 4 30 plus years , i have a 180 gallon african tank with a fx6 , checked into the siporax u recomend witch one is the best for me they have many different ones !!

Hi Mike,

A 180 gallon African tank sounds darn impressive. Do they have more than two types of Siporax now? Last I checked, there was only the “ceramic ring” style and small circular pellets for tiny filters. The ceramic ring style is what I use in my canister filter.

I Google searched “normal nitrate level in freshwater planted tank” and randomly picked your article. I was curious about a few things pertaining to nitrates and your article not only satisfied my curiosity but also taught me more about nitrates that I didn’t know. Thank you for a well written article that was easy to understand and really informative. My husband and I have multiple freshwater planted tanks (and an empty 40 breeder and 20 high still to setup), 3 betta tanks and love our fish. As knowledgable as a fish keeper may or may not be, you always have forgotten something, need refreshing or need to learn something new. I find so many times articles are either lacking or written for PhD level readers. That’s why I took time to praise your article because it was written perfectly (I also learned my nitrates are right on par for the one planted tank that was at 30 but can’t understand why all the others are lower, but since it’s normal level and my tank is fine no biggie I guess) and I enjoyed it. Yes I do love learning new fish related things… It’s how I keep my fish kids at their optimal selves lol! Take care!

Hi Megan,

It sounds like you have been bitten by the fishkeeping bug, the two empty tanks still to be set up is an obvious clue. It’s an addiction! Thank you so much for your kind words. It means so much to me that I could help!

In regards to the seachem prime product.
Could you carry on dosing 5ml every few days to help with levels or would this be harmful in the long term.
Also if you could would you need to use stability with it.

Hi Darren,

Seachem prime is just a temporary solution. It’s not going to work long term, your fish will end up getting sick. Water changes are by far the best way to lower nitrates.

I did a 40% water change, fish are still gasping nitrate still high, can I do another water change , when the fresh water first enters the tank gasping goes away for a while but comes back. Had tank up over a year first problem of this

Hi Joshua,

You can do as many water changes as is necessary (as long as you are conditioning the replacement water) You want to get the nitrates well under 40 and ideally under 10. Keep in mind that a 50% water change will roughly cut nitrates in half. So if your nitrates are 180, that would drop them to 90, which is still way too high.

It says clean the filter in your article of build-up, but I was under the impression that was where the good bacteria lived and not to do so. Our betta’s nitrate (20) and nitrite (.5) levels are both a little high. Should I be rinsing the filter when I change it out?

Hi Brianna,

It’s fine to rinse the filter in fresh water. Over time debris and gunk is going to get stuck in your mechanical filtration (typically a sponge) and reduce the flow rate anyway, so a quick rinse won’t harm it. I typically use the tank water I take out during a water change for the rinsing. The good bacteria is a stubborn clinger, and will stay put even through this, although ideally, you would want most of your beneficial bacteria to be living in a biomedia (like ceramic noodles) this makes cleaning and maintaining easier.

Any nitrite levels are a cause for concern and should be sorted out ASAP. Nitrates are generally fine until 40 ppm – weekly water changes should prevent them from nearing this.

Hi Adnan,

Thank you so much for the lovely feedback. I am glad I was able to help. Wishing you all the best with your fish keeping!

OK, trying to figure out what is up here.
1) Had a tank for a while, but it got moved, and thus redone. Not sure if I’m missing something that was working fine before or, more likely, what I think it is.
2) I have all guppies. But they’re a fertile bunch. So the tank has lots of fish in it. Many more than the suggested amount, but they are varying sizes, so that seems hard to quantify.
3) Water is clear but slightly brown in hue, and high in nitrates when tested. All other numbers (pH, Ammo, Nitrites) are not particularly out of whack.
4) I do feed the fish, but they always act as though they are starving — racing and grabbing what goes in before each other can do it. After 2 mins or so it’s virtually all gone.
5) Pleco to handle waste (not a final solution, I know).
6) Note: Water at new location is well-water through a softener, and is distinctly softer. I’m adding some pH reducer to new water to make sure it isn’t too soft. Metrics for softness are on the oversoft, but the overall pH seems about right. Just noting this for consideration.
7) A number of plants in the tank to help with the process, including some moss balls and dwarf grasses.

Things can’t be too bad, I just saw the first clutch of babies since the tank was relocated/rebuilt, so they’re not greatly stressed.

Just wondering what is the best thing to do/likely cause of the nitrates and (presumably nitrate-caused) brownish water.

Am I overfeeding even when there’s no leftover food?
I suspect the load is too high (too many fish) it’s a 10 gallon tank. I have a 30g. that I want to set up but have not yet had the time… I also want to buy and install a high-load shelf (500-lbs — water in tank should be less than 350# so I assume that should work ok with tank and equipment) so I can slide the tank forward out of its alcove so I can access it easier…

Any ideas/suggestions appreciated.

Hi OBloodyHell,

When you say “not particuarly out of whack” what are the results? These are the most important numbers and to gloss over them makes diagnosis near impossible.

Depending on the product, generally, pH reducer is the opposite of what you want, it makes water more acidic which does the by removing KH, or carbonate hardness

Unless you have a heap of poop fromy our fish, Brown water is most likely to be caused by tannins that come from organic matter? do you have any wood or dried leaves in the tank? Activated carbon will get this down.

However,the problem is most likely the overcrowding, plecos are waste machines and need a much larger tank.

hello ian! i am once again coming to you for help! so, i got fish, yay! and i was trying to be the diligent new fish owner and make sure to do my water change every month with general maintenance, so yesterday i tested my levels and noticed how surprisingly high my nitrates were. eek! i also noticed brown spots on my plants, umm that’s new. yesterday i did a 25-30% water change and did gravel siphoning. last night i also realize that the indicator light on my filter has never come on in the 4 months i’ve had it! i also then realize i need to change my filter every month! #neebie!. so today i change my filter and do another 50% water change via gravel siphoning. i read in various places that you don’t want to do a filter change, gravel siphoning and a water change all at once or it can muck up all your bacteria. sh#t.
i did not throw away my old filter, it’s behind my new one. i was afraid that with the water change and gravel siphoning i took away took much of my bacteria and if i throw away my old filter i’d lose everything. (remember it did take me 3 months to cycle my tank!) my nitrate is still around 20ish, so i think most of it is coming from my filter. (?)
my question to you- can i throw away my old filter now without throwing away most of my bacteria? (i also cleaned my filter, rinsed out the cartridges holding things in old aquarium water as well.) or should i wait a few days? what should i do?

Hi Valerie,

It’s been a while since we chatted, so you may have to remind me if I have asked this before (my old mind can’t keep up!) does your filter have a disposable cartridge? If so, you want to swap over to using a thick sponge and ceramic rings. Otherwise each time you throw out your filter, you throw out the good bacteria and need to cycle your tank again. You did the right thing by keeping it in there! To answer your question, you should keep your old filter in there for around a month, just to make sure the beneficial bacteria have moved to your new filter – I don’t want you to have to go through that big cycling process again!

On the nitrate issue, typically you would need to do a water change every 1 -2 weeks, a month is probably leaving it a little long. Not only does this help keep your nitrate down, but it allows you to catch issues like your filter not working, since you are working in the tank more regularly.

As for your nitrate levels, a 50% water change gets rid of roughly 50% nitrate. Lets use an example: if your nitrate was at 80, a 50% water change would roughly drop it to 40 (still too high) However, a 50% water change at 20 would drop it to 10 – which is perfect.

If you get your nitrate down under 10, you should be able to keep it manageable every 1-2 weeks. For instance, if you get your nitrate down to 10 and it rises to 20 over 2 weeks, a 50% water change will drop it back to 10.

However, if you were to do a 25% water change when it reaches 20, it would only drop nitrate down to 15. From here, the levels are going to get higher and higher because the 25% water change can’t keep up.

Please let me know if anything I have said here is confusing and needs explaining!

yes, my filter does have disposable filters. it’s a aqueon QuietFlow LED PRO Power Filter for my 20 gallon tank. i have heard people mention sponges, but i don’t know how they work. i would get rid of my filter? i do have a bubbler- it’s a little scuba person.


Hi Valerie,

You should be able to buy small pieces of sponge that can take the place of your disposable filter. Alternatively, there are “pre-filter sponges, that sit over the inlet of your filter, giving you more room for ceramic rings inside. The Sponge is essentially the first part of your filter that water flows through, it filters out bits of poop and other large floaty bits. If you have a disposable filter, there is probably some flossy like material in it. This is the same idea. Every few months, when the sponge looks really gunky, you rinse it in the water you removed from your tank during a water change. By doing this, sponge can last a year or more. As before, if you are making the switch, don’t remove the disposable filter for up to a month so you don’t lose the beneficial bacteria.

i have a 45 gallon tank with an aqueon 50 HOB filter. our nitrates are a little high. only have a carbon filter with a ammonia reducer pad in it. we’ve added 5 moss balls as recommended by the pet store. i try to keep up with the water changes and cleaning. but sometimes forget with having a pre-schooler running around. I’ve heard the Aqua Clear Fluval Power Filter is a good filter since it has separate bio bags and stuff … new to this kind of stuff. will getting this new filter help with maintenance if i forget one week. or will it make no difference since I’ve read that you must change every week no matter what… Help this not so knowledgeable fish owner learn new stuff. thanks for your time

Hi Jen,

I completely understand that a busy life can get in the way. Unfortunately, nitrates are the end of the nitrogen cycle. At this point, a water change is the best option for getting them back down. If you are finding them building up to quickly, then you can get around this by having less fish or a larger tank. In this case, you might be able to go 2 weeks without a change. But by and large, a new filter isn’t going to be the silver bullet you are looking for. Moss balls wouldn’t do much either – but they sure look pretty!

Also, in a healthy, cycled tank, your ammonia should be eaten by good bacteria as quickly as it is produced. If you remove the ammonia reducer, and you read ammonia levels higher than zero, then your tank likely isn’t fully cycled. Unfortunately, ammonia pads hide problems rather than solve them.

I have been having a nitrate problem for months. The nitrates are over 100 ppm, nitrites 0, ammonia 0. I do weekly changes of 40-50% and the nitrates won’t budge. I keep adding plants but they tend to die off. Even algae won’t grow. I would think that the plants would thrive or algae would take over the tank.I have tried chemicals, pads, adding a second filter…. still the readings stay the same. This take is over 30 years old.

Hi Thomas,

What is the max limit on your test kit? Most test kits only go so high. For example, if your max reading is 200 ppm but you ACTUALLY have 500ppm then a 40-50% water change may not get it down to readable levels. When this happen I usually recommend back to back water changes. The most extreme case I have seen needed 4 x 50% water changes.

I tried three different kits. One maxes at 160, the other two at 250. All show it as around 100. After yesterday’s water change it did drop down below 100. I am going to test again today and see what they say. The water is also a bit cloudy. I am using both a canister Cascade 1000 and Cascade 300 power filter plus a 8w UV filter. Ph is around 7 and I keep buying plants, but they mostly die off.

Weird, the API liquid test is showing 0 ppm but the test strips are showing 60 ppm. I tried the API test twice and both are now showing 0.

Hi Thomas, in cases like this, I recommend double checking with your local fish store. Have them perform a test on your water (take in a bottle) while you do the same test there with each of your test kits – this will help narrow down which one is correct.

The API testing was consistent with the others before the last water change, but then again the test strips keep showing the same results each time I do it. I would assume that the API test is more accurate.

It seems I made an error with the API test. I did not shake bottle 2. I retested but I am not really sure the best way to read it. If it is held close to the card it is reddish, but if I am in a lighter room or keep it about 1/4″ away it is orange. It is either 40 or 80 then.

For the liquid test kits, it’s best to test in natural day light (say, outside in the shade in the middle of the day) indoor lighting can really throw out the test colors – especially if your lights give off a somewhat orange glow.

Hello and Happy New Year. I find your articles very helpful since this is my first tank in about 15 years or so.
I have a 29 gallon freshwater planted aquarium. I have low to medium light plants: anubias, java fern, a type of crypt, and sword plants. I also have some frogbit which I keep corralled in an air tubing ring and thin out as needed so it stays put over a quarter of the tank.
My ammonia and nitrites are both 0. My ph is 7.4 and my nitrates were 10 before my last water change and now they are between 5 and 10 (20% water change). I have a led light for plants. I use easy green fertilizer, flourish root tabs, and flourish excel. I do not use CO2.
My question is, what is the best way to get my nitrates to between 10 and 30 ppm? I have a master test kit from API. I am not overstocked so should I add more fish? Should I hold off on water changes and keep testing the water until I get to 20 ppm? Should I fertilize more and keep an eye on the levels as I tweak the amount of easy green that I add to the tank?
I have 4 ottos that keep the algae in check and my four albino corys polish off the food that doesn’t get eaten as well as the sinking bug bites for them, i have 4 white cloud mountain minnows, and 3 gold ring danios.
Thanks in advance for any suggestion you might have.

Hi Sue,

Interesting question. SO in order to keep your nitrates between 10 and 30, you would generally just wait. Even in an understocked tank, your fish should eventually produce enough waste to reach these levels. From here, it’s deciding how much water to remove at each water change. This will take some trial and error. But the amount removed will remove a similar amount of nitrates. For example, a 50% water change will drop nitrate levels by *roughly* 50%.

I have a 60 gal freshwater tank, HOB filter and undergravel filter w/ 2 powerheads to circulate water. I thought I cycled my tank with a normal cleaning, which I do every 2 weeks, but both the ammonia and nitrite levels are OK and never spiked, but the nitrate levels spiked to around 80. This has been going on for around 3 weeks now, with numerous 33% water changes and adding Fritzzyme 7 bacteria each change. The nitrate levels have not gone down. I also changed the carbon filters. I have some plants in the tank, but not plenty. I have been having aquariums since the 80’s. This particular tank has been up and running for over 4 yrs and have not experienced this problem in the past. Someone recommended removing the drift wood I have in the tank. Have you heard of this being a possible source of nitrates? I have been having these for a long, long time.

Hi Chris,

You could try removing the drift wood, if something is rotting inside then it could lead to an increase in nitrates. Unfortunately, finding the cause is often a case of trial error remove -> water change -> retest.

I have a 20 gallon freshwater tank with, now at present, 2 good size angels and a small catfish type bottom feeder. There may or may not be a loach hanging around. There used to be 3 more fish in the tank but it has been 6 months or more. I’ve done several 25-35% water changes. I have an Aqueon canister filter that does 200 GPH with a polishing filter that the water flows through back into the tank. I have the API chemistry set test kit. I have alkaline well water. My ammonia has alway been slightly positive. Nitrites have ALWAYS been negative. Nitrates have never been below 40 and I probably cheat on that reading to think they aren’t higher than 40. Well water nitrates are negative. My wife feeds the fish and I harp on the over feeding thing. Any thoughts?

Leave a Reply

Your email address will not be published.