Fish tank nitrite is a simple chemical made up of two atoms of oxygen combined with one of nitrogen, hence NO2. It can be represented simply by Diagram 1 above.
Fish do not directly produce nitrite. Rather it is primarily an end product of the first stage of nitrification that occurs within a biofilter. Fish constantly excrete ammonia as part of normal metabolism. Equation 1 shows the conversion of ammonia to nitrite performed by specific nitrite oxidising bacteria.(NOB)
Like ammonia, nitrite can prove toxic if allowed to accumulate. Nitrification is constantly producing fish tank nitrite so it is important not hinder its conversion. This is one of the many stages that occur as part of the Nitrogen Cycle.
Secondary Source of Nitrite
Interpretation of elevated nitrite levels can be a little tricky as it can also accumulate as a result of the denitrification of nitrate. This largely occurs under conditions of low dissolved oxygen within a biofilter.
Toxic effects of Nitrite
Nitrite is something that should not be allowed to accumulate even though it is less toxic than ammonia. It can severely interfere with the oxygen carrying ability of fish flood. It does this by oxidising the iron within the haemoglobin rendering it unable to bind with oxygen. As a consequence the blood turns brown instead of a healthy bright red through the formation of methaemoglobin. ‘Brown blood disease’ is a common and descriptive term used to describe this condition.. There is a similar mechanism occurring in carbon monoxide poisoning.
If your fish appear to be gasping at the surface but you know that the dissolved oxygen levels are good then nitrite poisoning might be a possible cause. Other indications include fish ‘flashing’ (i.e. quickly rubbing themselves against the pond surfaces) and lying still on the bottom.
Controlling Fish Tank Nitrite
As with ammonia, the most practical method of keeping nitrite at acceptably levels is to utilise a biological filter. During the second phase of nitrification another type of bacteria, of which Nitrobacter is just one example, feed on the nitrite that is produced from ammonia by ammonia oxidising bacteria (AOB) The end product of nitrification is nitrate and this is considerably less toxic than both ammonia and nitrite.
Recommended safe levels for nitrate do vary but generally speaking less than 50 mg/l is fine. Some aquaculture systems have operated with levels exceeding 1,000 mg/l without any detectable problem.
The Effect of Chloride on Nitrite Toxicity.
The toxicity of nitrite is highly dependent on the chloride concentration. This is due to the ‘ion pumping’ that occurs within specialised cells. When sufficient chloride ions are present fish will not accumulate so many nitrite ions. The addition of salt (NaCl) may, as a consequence, significantly reduce toxicity levels. A 0.3% solution is a good starting point. This represents 3kg of salt per 1,000 litres. One more point is to use PVD (partially vacuum dried) salt as this does not contain poisonous anti-caking additives. This however is no substitute for an effective biofilter. Generally speaking the conversion of nitrite to nitrate is a fairly simple and rapid process that occurs faster than its formation from ammonia. Denitrification however, may also cause elevated nitrite levels.
Safe levels of fish tank nitrite
The Ornamental Fish Trade Organisation recommend a maximum safe level of 0.2 mg/l nitrite. Compare this to the recommended safe level of 0.02mg/l of unionised ammonia. Different fish species and different culture conditions will always play a large partin determining a safe level. Warm water species such as catfish, carp and tilapia are more tolerant of poor water quality than cold water species such as trout and salmon. Marine species are less tolerant than freshwater. The sensibly approach is always to keep fish tank nitrite as low as possible, monitor regularly, reduce stock and carry out water changes whilst the problem is investigated further.