Nitrates can kill fish

For anyone new to keeping fish there is a lot of information about keeping fish safe from the effect of ammonia and nitrites in the water. There is less written about the potentially deadly effects of nitrates in the aquarium.

Nitrate is less toxic to fish than ammonia and nitrites, but it can still kill your fish.  This is better known as Nitrate Poisoning and there is also Nitrate Shock.

Nitrate is an inorganic compound composed of one atom of nitrogen (N) and three atoms of oxygen (O); the chemical symbol for nitrate is NO3. 

Fish digestive wastes can build up in a tank if it is not maintained and cared for properly, dirty filters, decaying food and plant material, over-feeding and overstocking the tank, all contribute to increased production of nitrates.  Over time high levels of nitrate can build up which is bad for the fish and the plants in the tank.

The result of a build-up of nitrates is that fish are more susceptible to disease, it lowers their ability to reproduce and can reduce their growth.  High levels of nitrate can eventually lead to bacterial and fungal infections such as Fin-rot, Whitespot and other diseases that take advantage of lowered immunity.

Nitrate poisoning

This occurs with gradually rising nitrate levels over a period of time. Regular tank maintenance will prevent this happening.

Let’s be very clear about nitrate build up and poisoning – if you do not take steps to reduce nitrates, the effect can be fish death.

Not all fish react the same to nitrates, some will be affected by levels as low as 20 mg/l, while others will show no symptoms until levels have reached 200 mg/l.  Young fish are affected at much lower levels.

Note: The federal standard for nitrate in drinking water is 10 mg/l nitrate-N, or 50 mg/l nitrate-NO3, when the oxygen is measured as well as the nitrogen.  Unless otherwise specified, nitrate levels usually refer only to the amount of nitrogen present, and the usual standard, therefore, is 10 mg/l.   You need to be careful when adding water to the tank that it is suitable.  If you want more information on tap water limits follow this link to the EPA page – Click Here.

Most beginning aquarist will use tap water unless they know there is a reason to not use their tap water, such as the local municipal water supply is known to be contaminated with some toxin. Tap water usually has controlled properties and tends to be reasonably chemically stable.  Before adding water, test it for nitrates so you know if the levels are unusually high in your water source. If nitrates are above 10 ppm, you should consider other water sources that are free of nitrates.

Nitrate shock 

Shock happens when fish are exposed to a much higher level of nitrate, or when nitrate levels suddenly drop.  Some aquarists say that a sudden or rapid reduction of nitrates is just as harmful to the fish as high nitrate levels.  But not everyone finds this to be true, however, I need to warn you that this may be a problem.  Just like nitrate poisoning, immature young fish and certain species, Discus being notable, are more sensitive to sudden changes in nitrate levels.

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Algae loves nitrates

Increased levels of nitrates promote algae growth.  This can cause undesirable algae blooms in new tanks.  Although plants use nitrates, if the nitrate level rises faster than the plants can use them, the plants can become overgrown with algae, causing their death.

Target nitrate levels

The lower the better.  Nitrate levels are generally low naturally in fresh water, subject to it not being contaminated with plant and animal debris, generally well below 5 mg/l. (5 ppm)

In freshwater aquariums it is recommended to keep nitrates below 25 mg/l and certainly never over 50 mg/l.

Saltwater aquarium are different, some experts say that you can have levels slightly higher up to 40 mg/l for fish but corals and other invertebrates will suffer.  It is best to consider 25 mg/l as the top limit.  Please be aware that if you have a reef tank with invertebrates, you will want to keep the nitrate below 5 mg/l for their good health.

If you are breeding fish, or are suffering with algae growth, you must keep nitrates below 10 mg/l. (10 ppm).

Steps to reduce nitrates

When you realise your tank nitrate level is too high, you need to establish the actual cause.

Questions to ask yourself;

  • Is your tank overstocked?
  • Have you kept up with your partial water changes?
  • Did you clean waste from the gravel?
  • Are you over feeding?

If you have not been maintaining your tank correctly, then a few steps can easily get you back in control.  Starting by changing the above issues.

  • Water changes – Regular water changes with water that has little or no nitrates will lower the overall nitrate level in the tank.
  • Keep the tank clean – Cleaner tanks produce less nitrates.
  • Don’t overfeed the fish – This is a significant contributor to excess nitrates.

However, if you have been looking after your tank with good maintenance and tank husbandry then the level of nitrates in the tank can never realistically be lower than that of the water you put in it to start.

If the water from your tap was high in nitrates then you must treat it before using it and take further steps for the water already in your tank.

  • Adding more plants – these will consume more of the ammonia produced by the fish before it is converted to nitrate can lead to dramatic change. Floating plants such as Duckweed are good.
  • Use nitrogen removing filter media – Instead of an expensive or special filters, (conventional filters do not harbor the bacteria that remove nitrates) use special media in the filter you have. Nitrate absorbing sponges can be used in your filter, but must be regularly replaced as they become exhausted. Used with other methods the net result will be less nitrates at levels you want to keep.
  • Canister filters – these can be used as they have a biological filtration mechanism and have the advantage that they do not need to be regularly replaced or recharge.
  • Liquid additives – there are many, they contain small beads impregnated with nitrate-eating bacteria. Because these bacteria are typically anaerobic in their action, their life is limited and the treatment needs to be repeated weekly.  It is a very simple treatment to use as it is added directly into the tank, but there is an ongoing cost of regular treatment.

Water should be tested weekly using an aquarium nitrate test to monitor the level.  As a simple rule-of-thumb the level should be kept as low as possible: always below 40 mg/l and preferably below 20 mg/l.


11 thoughts on “Nitrates can kill fish”

  1. Thank you for a well written and easy to understand article. I appreciate that you gave nitrate levels for ideal tank health, realistic levels most fish can tolerate, as well as what levels are high enough to cause potential issues. You explained where nitrates come from and how to reduce the levels in an aquarium nicely. Pointing out what levels were potentially in tap water was a bonas – I’m off to test my tap water now.

  2. “Young fish are affected at much lower levels, as are saltwater fish”

    This is incorrect. Saltwater fish have a nitrate tolerance between 50-100 times that of freshwater fish.

  3. I didn’t know that plants could consume for ammonia – been looking for a reason to get my son some more plants for his first tank!

    Are there any simple test strips that can test the Nitrate levels?

  4. Lost some fish a few years back from nitrates – now I’m certainly more diligent about preventing this from happening. I don’t trust my tap water anymore.

  5. Does anyone else get nitrates and nitrates confused with eachother? I have to literally look it up sometimes, like which one is the one that’s more harmful for the fish.

  6. I always remember the change out the water often but for some reason I have a hard time remembering to clean the gravel too. Now I feel more motivated to do it! Nitrates are nothing to play around with.


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