Nitrate Poisoning in Aquarium Fish: Causes and Cures


If you notice that your pet goldfish is grotesquely curled up then it’s definitely time to reassess the water quality in your aquarium.

A timely reaction is crucial now, however, you should know how to gradually lower the high nitrate levels in the fish tank in order to avoid causing further stress to your aquatic pets.

What you’re witnessing signals that the sickness is in its advanced stage. Your fish is experiencing poisoning caused by elevated nitrate content in the aquarium, which usually takes place over time unlike the immediate effect of ammonia or nitrite.

Any form of irrational behavior in fish is to be considered as a potential threat to their health.

What causes Nitrate poisoning in aquarium fish?

The build-up of Nitrate or NO3– in the aquarium is the result of the nitrification cycle that naturally occurs in the water. However, the NO3– ions are not used by aerobic water bacteria and remain in the system until physically removed. This leads to long-term permanent damage in fish and you should react as soon as you notice ill behavior. Here’s the reason behind nitrate poisoning in aquarium fish:

Nitrate poisoning in fish is caused by long-term exposure to between moderate and high levels of Nitrate contents in the aquarium water. If 20 or more PPM (parts per million) of Nitrates are continuously present in the water, the fish will start showing signs of nitrate intoxication. A sudden rise of nitrates is also harmful and potentially lethal to aquarium fish. The most common reason behind nitrate poisoning is the unregulated build-up of dissolved organic compounds in smaller fish tanks caused by overfeeding or overcrowding.

Elevated contents of Nitrate in aquarium water can kill both freshwater and saltwater fish over time or during a spike.

It should be noted that some species of freshwater fish are more sensitive to nitrate than others. More fragile fish like the Otocinclus, Stingrays, Angelfish or Discus should be kept in aquariums with no more than 10 ppm of nitrate in the long run.

An increase in body mass and adaptation over time may reduce the sensitivity to nitrates.

As long as the build-up is gradual freshwater fish could tolerate up to 50 ppm of nitrate, where saltwater fish will show no signs of poisoning with up to 40 ppm for a limited period. At such levels, however, fish will inevitably start to show signs of sickness over time.

Still, even lower levels of nitrate can be fatal for fish fry and most invertebrates. Fish grow the fastest in the juvenile stages of their lives. Higher nitrates could potentially limit a fish’s maximum size if it’s grown from fry in such conditions.

Marine fish are relatively more susceptible to nitrate intoxication than freshwater fish.

A level of 20 ppm should not be exceeded at all times in saltwater fish tanks in order to minimize the adverse effects of long-term exposure to nitrate.

In coral reef aquariums, nitrate is to be kept at an absolute minimum, preferably at below 2 ppm, because at higher levels most of the more sensitive corals will perish.

Fish will start showing symptoms of poisoning after long-term exposure to moderate concentrations or after a sudden spike of Nitrate levels.

Signs and symptoms of a Nitrate intoxication in fish

In the freshwater fish keeping hobby, there’s a popular belief that the presence of nitrate in the aquarium’s water is not particularly harmful.

For this reason, many would automatically exclude it as the possible reason behind the symptoms that I’m about to list.

Truth is, however, that nitrate is relatively harmless, but only when compared to the highly-toxic ammonia and nitrite.

Both ammonia and nitrite can cause fish deaths almost immediately after their appearance, unlike nitrate.

This, however, does not mean that nitrate contents won’t endanger aquarium fish, be it over time.

That being said, here are the signs and indicators of nitrate poisoning in aquarium fish:

  • Lethargic behavior and lack of energy;
  • Weak or no feeding reflex;
  • Random erratic movements;
  • Side swimming;
  • Upside-down swimming;
  • Fast movement of the gills as if the fish experiences shortness of breath;
  • Lying on the bottom of the aquarium;
  • Fading color and a pale look;
  • A bent body with a curved spine where the fish looks curled up;

Here’s a photo of a fish with a bent spine which signals a severe poisoning from high nitrate in the aquarium water:

By iknowcraig

If the nitrates did not violently spike overnight, then the long-term exposure has started to take its toll.

When the signs start showing it means that the fish have already suffered for some time.

However, it should be noted that most aquarium fish will adapt to poor water quality, at least to a degree.

This is most obvious in a fish tank that has been running for some time and suddenly its aquatic inhabitants start exhibiting the aforementioned symptoms without an obvious change in the water parameters.

Even if a single fish shows a sign of poisoning it means that the water is polluted and the parameters need immediate adjustment.

Take a look at this photograph of a goldfish poisoned by elevated Nitrate, that displays a curved body in a U-shape:

By Jake0743

Another way you could tell that nitrate is slowly killing your fish is upon the addition of new tank mates.

If the new fish shows any or a combination of the signs, despite being quarantined and a lack of aggression from its tank mates then it may be that you’re running an aquarium that’s too high in nitrate.

The long-term inhabitants have gradually adjusted to the higher nitrate levels over time, but the new addition immediately starts showing signs of shock.

Other known ways of how elevated nitrates affect fish are a weakened immune system, lowered chance of reproduction and swim bladder disease. The latter causes imbalanced swimming where the fish floats to the top and swims sideways or even upside-down.

The Conservation Fund’s Freshwater Institute conducted an experiment to show the significant correlation between NItrate-rich aquarium water and swim bladder issues.

Here’s a visual representation of the correlation:

Unfortunately, by the time the symptoms become apparent, there’s already a degree of permanent damage.

Though fish can recover from poisonous nitrate levels they will likely become very sensitive to nitrates in the future.

In the most severe cases, a fish that’s poisoned can die in less than 24 hours.

For this reason, you should react quickly and start immediate treatment as soon as you notice the problem.

How to treat the affected fish?

Your first course of action upon spotting any of the aforementioned symptoms is to perform a water test by using a reliable liquid test kit.

Water test strips can be quite inaccurate and are not recommended when you want to assess the situation properly.

A very popular choice for a good water test kit, which I also approve, is the API Master test kit (link to see it on Chewy.com).

This kit has a freshwater, a saltwater and reef version so make sure to get whichever fits your setup.

After performing the water test, you’ll be able to establish if the poisoning was caused by a sudden spike in Nitrate or long-term exposure to lower levels.

If the readings fall within what you consider “the norm” for your aquarium, then your fish have likely been gradually intoxicated.

Depending on the species you keep and how long you’ve kept them, this could even be a reading of 20 ppm.

If, however, the test shows alarmingly high nitrate levels, compared to what your fish tank usually maintains, then it’s safe to say that the poisoning has occurred in the past 24 to 48 hours.

Ideally, in both cases, you’ll want to lower the nitrate content in the aquarium’s water to below 15 ppm and in the case of more sensitive species of fish – to below 10 ppm.

Here are instructions to treat fish that have been poisoned by high nitrate:

To initially treat fish that have been poisoned by nitrate and help them survive the toxic aquarium water you should perform a large water change of up to 40%, which will dilute the pollution. This is more of an emergency cure and it should be viewed as a temporary solution to prevent or put a stop to fish mortalities. By doing so you’ll buy yourself time to later evaluate the real reason behind the elevated nitrate levels.

Test the fish tank’s water and the results will give you a rough estimation of how large a water change you’ll need to perform.

A general rule of thumb here would be that interchanging 50% of the aquarium water will remove 50% of the nitrates.

For example, if a freshwater fish tank’s water reads 30 ppm of nitrates then you should exchange roughly 30 to 35% of the total water volume to bring them down to below 20 ppm.

If the water tests show 100 ppm of nitrate content then you’ll want to perform an 80% water change in order to bring them down to 20 ppm.

However, and this is important for the safety of freshwater aquatic pets, I do not recommend interchanging more than 40% of the total water volume of a fish tank.

This is because…

GRADUAL water interchanging is key

Because of the relatively low toxicity of nitrate, all aquarium fish will adapt to its constant presence in the water, at least to a degree.

For this reason, the sensitivity to nitrate fluctuations goes both ways.

Same as with rapidly elevating levels of nitrate, drastically reducing them will put a fish under immense stress.

Such stress can weaken the fish’s immune system even further and impair its osmotic regulation.

Osmoregulation is the ability of fish to regulate their body fluids, which is crucial when it comes to swimming.

This is often the mysterious reason behind swim bladder disease worsening after a fish keeper rushes to change a large portion of aquarium water at once.

The bladder is supposed to be filled with gas, but during disturbed osmoregulation it fills up with liquid, which then causes buoyancy issues and eventually death.

Impaired osmoregulation causes swollen internal organs, which is the reason behind a bloated fish that has survived a nitrate intoxication event. This type of bloat is often observed in aquarium fish kept in smaller tanks such as Betta fish, because the less volume of water is easier to pollute.

To prevent the shock a large water change would cause to a fish during nitrate poisoning you should not change more than 40% of the total water in the aquarium per day. Spread this 40% in multiple smaller water changes of 5% for every 1 hour.

So, if circumstances demand it to perform the largest water change your fish can endure for the day, the whole procedure should take around 8 hours.

You can interchange the remaining water in the following day.

Note that in the most extreme cases where aquarium water contains more than 100 ppm of nitrate you should not remove more than 50 ppm per day. This is, again, to prevent ion imbalance in the water, which could lead to impaired osmoregulation in fish.

This means that if the aquarium has 200 ppm of nitrate, the maximum volume of the water change should not surpass 25%, which equals 50 ppm of nitrates.

However, if the fish tank has been shown to contain 60 ppm of nitrate, don’t go over the 40% mark of exchanged aquarium water.

An 8-hour long period of small water changes may sound daunting, but if you are fond of your fish, it’s the best possible approach to a safe emergency treatment.

Before you rush to the siphon, however, you should test your tap water.

Tap water as the culprit

In some geographical regions, things like fertilizer runoff and the erosion of natural deposits can leak nitrate in drinking water.

The EPA (the United States Environmental Protection Agency) has set a hard limit of nitrate content to drinking water supplies.

To be compliant, each water facility should not allow more than 10 ppm of Nitrate-Nitrogen in the water they provide for drinking.

In the context of fish keeping, we must convert this number by dividing it by 0.2258, because nitrate-nitrogen only reports the nitrogen content in a nitrate ion (NO3-), which is roughly 22.58%, with the rest being oxygen atoms.

We then find out that tap water in the US could legally contain up to 44.29 ppm of nitrate.

In fact, tap water all over the world could contain nitrate, depending on the local regulations.

For this reason, you should definitely test your tap water for nitrate because it could actually be behind the build-up from the beginning.

If tap water turns out to be the culprit you’ll need to consider a different source for aquarium water changes.

A good recommendation would be RO/DI or Reverse Osmosis / Deionized water which is purified and contains almost nothing but H2O molecules.

This type of water requires remineralization, because freshwater fish get their minerals through their skin, unlike humans.

RO/DI water is heavily used in coral reef aquariums where the presence of nitrates can be detrimental to the more sensitive coral species. In these setups, the aim is to keep nitrate at below 2 ppm.

You can find RO/DI water in your local pet store where they sell it by the gallon.

You could also invest in an RO/DI filter, which will let you purify your tap water at home.

An RO/DI water filter requires a higher initial investment but pays for itself with time and is the cheaper option in the long run.

You could order some pretty decent ones for under $80 on Chewy.com alongside an inexpensive remineralizer such as Seachem Stability and never worry about nitrate toxicity in the aquarium again.

I can save you some time and point you to one such filter that I’m personally happy with, but you can browse around and do your own research.

Other possible causes for high nitrate and how to permanently fix the issue?

Because nitrate is rarely converted to anything in a freshwater aquarium it remains in the system.

Any organics that end up in the system will eventually become nitrate.

The most common reasons for poisonous levels of nitrate build-up are:

  • Feeding too often and more than the fish can eat.
  • Overcrowding the aquarium beyond its bioload capacity
  • Having a tank that’s too small to adequately house fish or anything below 3 gallons for a single small Betta fish
  • Keeping messy fish species such as Goldfish
  • Neglected aquarium water maintenance
  • Old filter media
  • The lack of any live aquarium plants

Aggressively changing water is a short-term fix to immediately reduce the physiological stress the nitrate poisoning has caused to the fish.

For effective long-term solutions after you’ve handled the emergency I strongly recommend checking my guide on permanently lowering the high nitrate levels in a freshwater fish tank.

Visit the link to learn about various methods and ideas on commercial and natural nitrate-removing filter media for a freshwater aquarium.

To learn more about managing healthy levels of aquarium nitrate in a saltwater aquarium click here.

Dealing with the underlying reasons behind the contamination would ensure the problem is permanently resolved.

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Sara Steed

Hello. Not sure if you’ll get this, but I sure hope you do. I’ve learned SO much and I’m very grateful for your information. Our first fish, a Beta, he has always done great! We are first time fish owners. He is in a 3.5 gallon tank. Has a couple live plants and a couple things my toddler picked out to be in there with him. He has always been a happy fish! Every time we clean the tank we put everything back just as it was! He has always had his same sleeping spot, and his favorite color in… Read more »

moiz raza

Many fishkeeping enthusiasts debate the “best” way to cycle a tank properly. The key to doing this right is to just be very piecemeal in the beginning.