Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Nitrate in the aquarium

Goldfish, Carassius auratus auratus
Goldfish, Carassius auratus auratus


Nitrate is a compound of nitrogen and oxygen, NO³. It is made by nitrifying bacteria which use ammonia NH³ for energy and in doing so they oxidise it in to nitrite NO² this is used as a source of energy by another species of bacteria and oxidised further in to nitrate, this is usually the final stage in the breakdown of ammonia.

Fish constantly excrete ammonia and ammonia is also released by bacterial activity when breaking down waste. This means that nitrate is constantly rising in an aquarium.

The best way to prevent this build up from becoming dangerous is to make regular partial water changes which should be sufficient in volume and regularity to keep the nitrate within safe limits for the fish.

Nitrate levels should be maintained as low as possible, some sensitive species are adversely affected by nitrate levels of just 48mg/l while others tolerate higher levels before being affected. With this in mind the maximum safe limit should be regarded as being in the region of about 25mg/l for a fresh water aquarium and even lower for a marine reef system.

Nitrate effect on fish

If the fish are exposed to elevated levels of nitrate over prolonged periods of time it will adversely affect them in several ways.

Exposure to high levels of nitrate over a long period will compromise the fish's immune system. Scientific studies showed that at levels of 200mg/l of nitrate for six weeks made the fish's immune system respond only at 48% compared to fish which had been living in water with much lower nitrate levels.

The fish's blood also underwent changes with an increase in immature blood cells found in their blood stream.

After three weeks of living in water where the nitrate level was 200mg/l the fish appeared to go blind and some died.

When the fish were examined, it was found that both liver and kidneys had been adversely affected.

  1. Affects antibody production
  2. Increased number of immature red blood cells
  3. Lowered level of mature red blood cells (anemia)
  4. Higher count of monocyte (a specific white blood cell)
  5. Higher count of neutrophil (a specific white blood cell that is especially destructive to microorganisms)
  6. Higher count of TLC - Thrombocyte-like cell (a blood cell of nonmammalian vertebrates that promotes blood clotting)
  7. Higher levels of creatine (A nitrogenous organic acid found in muscle tissue that supplies energy for muscle contraction)
  8. Higher calcium values in the blood
  9. Lower Chloride values in the blood
  10. Autopsy revealed damage to the spleen, liver, and kidneys

Ref: A study of nitrate toxicity

Nitrate tolerance

Different fish have different tolerance levels to nitrate.

  • Chinook Salmon, Rainbow trout and cut throat trout have probably the lowest tolerance and some juveniles died when the level of nitrate exceeded 7.6 mg/l
  • Goldfish can tolerate levels in excess of 1,000 mg/l with most other fish falling between the two.
  • The maximum recommended safe limit for aquarium fish is 25mg/l although lower is better.


Nitrate: Is often thought of as being completely non-toxic but this is not the case. Nitrate is the least toxic of the nitrogen compounds in the aquarium but it is still toxic.