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 filter bacteria.(NOB)
Like ammonia, nitrite can prove toxic if allowed to accumulate unhindered. Unlike nitrification it is not affected by pH. Nitrification is constantly producing nitrite so it is important not hinder its conversion. This is one of the many stages that occur as part of the Nitrogen Cycle.
With every new tank there hasn’t been sufficient time for the nitrifying bacteria to have increased in numbers to allow complete nitrification. The nitrate level will start to rise as soon as the processs has started but nitrite levels may continue to also rise until the biological filter has fully matured. Fishless cycling is a popular technique to mature a biological filter of a new tank prior to stocking it with fish. Water chemistry must be assessed during this phase using readily available test kits.
Secondary Source of Nitrite
Interpretation of the origin 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. Such conditions are common in many fish systems either on the pond bottom or within the biofilter.
There is also some research indicating the ultra violet (UV) light can breakdown nitrate into nitrite. This is something that is not generally recognised but may well be significant in some systems.
Toxic effects of Nitrite
Nitrite is something that should not be allowed to accumulate even though it is less toxic than ammonia. It has a very wide range of negative affects that include;
- A reduction in the blood oxygen carrying capacity
- Interference with water balance
- Interference with osmoregulation
- Disruption of the endocrine (hormone) system
- Suppression of the immune system
- Reduction in reproductive performance
Nitrite 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.
As far as ionic balance is concerned nitrite enters the fishes body via an active uptake Cl-/HCO3_ exchange mechanism.As a result an increase in the nitrite level in the blood tends to be associated with a
- a drop in chloride ions (CL-)
- an increase in potassium ions (K+)
- a drop in sodium ions (Na+)
The K+ ions are released from skeletal muscle and red blood cells thereby leading to an increased level within the blood plasma.
Symptoms of Nitrite Poisoning
Nitrite poisoning can take many forms and, consequently, can be detected by a variety of symptoms. Many of these are not restricted to nitrite so it is important to carry out appropriate water testing.
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. Confirmation can be obtained using a simple nitrite test kit. Other indications include fish ‘flashing’ (i.e. quickly rubbing themselves against the pond surfaces) and lying still on the bottom.
- General lethargy
- Hovering at the pond bottom
- Hanging at the surface close to the water intake
- Gasping for air at the surface
- Swollen, brown gills
- Reddening of the skin
- Gill clubbing
- Fusion of gill lamellae
- Liver damage
As with ammonia, the most practical method of keeping the nitrite level at an acceptably levels is to utilise a biological filter. During the second phase of nitrification another type of nitrifying 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, fortunately, this is considerably less toxic than both ammonia and nitrite. The reaction shown below does require the presence of some oxygen.
If the filter bacteria that are growing on the filter media are compromised in any way, partial water changes would undoubtedly help to maintain better water quality. Once effective biological filtration has been re-established the Nitrogen Cycle continues and these water changes can be reduced.
An additional method of control, other than biofiltration. could involve the introduction of ozone. Probably only viable for larger systems it does however offer a viable alternative if biofiltration does not appear to be effective.
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. Bear in mind though that it is highly likely that different species of fish exhibit different tolerances to nitrate.
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 aquarium 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 the nitrite level to increase.
Safe levels of fish tank nitrite
High levels of ammonia and nitrite can be toxic. 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 part in 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 tank nitrite levels as low as possible, monitor regularly, reduce stock and carry out water changes whilst the problem is investigated further.