Nitrite Toxicity

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The nitrogenous cycle converts toxic nitrogenous waste into relatively non-toxic compounds and is carried out by a mixed population of bacteria present in the biological filter and in surface biofilms. Ammonia is oxidised first to nitrite then to nitrate.

Both ammonia and nitrites are very toxic to fish, nitrate is less toxic but should not be left to accumulate to high levels.

Nitrite is transported across the gills where it enters the bloodstream and oxidises haemaglobin to methaemaglobin. This leads to tissue hypoxia.

Nitrite toxicity is particularly a problem in newly-established recirculating systems, in which insufficient numbers of bacteria are present in the biological filter. This can also be a problem when the biological filters have been damaged by the interruption of water supply, excessive cleaning or antibacterial administration.

Species susceptibility to nitrite is variable and is linked to chloride uptake by the gills. Nitrite competes for chloride absorption, therefore fish with a high chloride uptake such as the rainbow trout, the perch, the pike and the catfish are more susceptible to nitrite. In contrast, the largemouth and smallmouth bass, bluegill and green sunfish are resistant to high concentrations of nitrite.

Clinical Signs

Fish with nitrite toxicity will present with tan to brown-coloured gills and show signs of hypoxia such as gathering at the water inlet or surface.

Changes in oxygen affinity and blood pH increase the movement of oxygen to the swim bladder. This can lead to buoyancy problems.


Diagnosis relies on the measurement of nitrite levels in the system or tank water, and some gross evidence of methaemaglobinaemia.

Toxicity levels vary between species of fish, but it is preferable to keep levels as low as possible in all cases.

Treatment and Prevention

Nitrite is much less toxic when chloride is present, possibly because chloride inhibits nitrite absorption across the gills.

Sodium chloride is the most available and cheapest source of chloride.

Haemaglobin levels should return to normal within 12-24 hours. If severe anaemia is present, recovery may take weeks.

The original cause of the nitrite toxicity should be resolved. Levels of nitrite in the water can be reduced through water changes, biological filtration and decreased feeding.

Prevention involves ensuring an adequate level of chloride in the water at all times. Most waters already have enough chloride and prophylactic treatment is unnecessary.

Nitrite Toxicity Learning Resources
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Ornamental Fish Q&A 13


Noga, E. (2010) Fish Diseases: diagnosis and treatment John Wiley and Sons

Gupta, R. (2007) Veterinary toxicology: basic and clinical principles Academic Press

Dietrich, H. W. (2009) Essential Zebrafish Methods: Genetics and Genomics Academic Press

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