Longnose hawkfish, Oxycirrhites typus
Nitrite is arguably even more toxic than ammonia to fish. It arrives in the aquarium as a result of oxidation of ammonia first to nitrite and then to the less toxic nitrate. In a well looked after mature aquarium the nitrite is kept to undetectable levels thanks to a healthy population of nitrifying bacteria and so it poses no threat to the livestock. When something goes wrong with a filter, the wrong medication is used where the nitrobacter are killed off or if fish are added to the aquarium to soon before it is fully mature there will be a rise in the nitrite level in the aquarium which then puts the lives of the fish in great danger.
Nitrite is being produced night and day in an aquarium by the bacterial breakdown of ammonia which itself is produced by the fish and from the breakdown of organic waste.
Normally it is kept at levels so low it can't be detected using an aquarium test kit. The actual level of nitrite in fresh water is typically 0.005mg/l and isn't a problem for the fish at this very low level.
If for some reason the levels of nitrite rise the fish are in immediate danger because nitrite is carried across the fish's gills where it comes in to contact with the fish's haemoglobin and forms bonds with the fish's methemoglobin.
This cause major problems for the fish because it is the haemoglobin which carries oxygen around the body, methemoglobin on the other hand does not.
Some fish are quite tolerant of nitrite while others can be killed with exposure to just 0.29 mg/l of nitrite over a 96 hour period of time.
Nitrite is actively taken up by freshwater fishes and the levels in the fish can reach ten times that found in the environment. Once the fish is exposed to nitrite and the blood of the fish contains a high percentage of the haemoglobin has been replaced with methemoglobin the fish will become hypoxic. This leads to the first symptom of lethargy. The fishes muscles cannot operate properly for sustained periods with out oxygen. At the same time growth is reduced by up to 44%. If the exposure to nitrite continues the fishes tissues could be affected and this shows itself as what appears to be bruising on the fish. Eventually some tissue will become necrotic and will slough away leaving large ulcers on the fish. This in turn causes extra problems with osmosis and secondary infections.
If a fish suffering from exposure to nitrite is scared or forced to exert itself it could die very quickly from anoxia.
The first priority is to remove the fish from the nitrite. An immediate water changes using nitrite free water should be carried out to reduce the level of nitrite in the water to >0.2 mg/l. This is regardless of how much water has to be changed, because the nitrite is so toxic that it is more damaging to leave the fish exposed to it than it is to changing water chemistry.
Secondly adding a little salt to the water will also be of great benefit because the chloride in salt reduces the up take of nitrite and so the percentage of methemoglobin in the blood will be reduced. (Salt calculator) It only needs a tiny amount of salt too, just 20mg/l is sufficient to have this effect. Or just 1/2 level teaspoon of salt will treat 125 litres of water.
This is why marine fish are far more tolerant of nitrite than fresh water fishes and why fishes from very soft water are the ones generally most at risk.