This is an environmental disease known commonly as Brown Blood Disease.

Nitrite poisoning comes just below ammonia poisoning as a major killer of the cultured fish family. If you have been having problems with ammonia, then it is almost certain that nitrite levels in the water will rise at the same time. This means your fish will not only be at risk from the ammonia, but from nitrite as well. The levels of nitrite rising happens after the ammonia levels rise.


The symptoms to look out for are:

– Your fish gasping for breath at the surface of the water

– Fish trying to get more oxygen by staying near water outlets.

– Their gills start moving rapidly

– Gills change colour by turning brownish


Lethargy and sluggishness

Always stock up your aquarium slowly and never over-feed the fish. Remove all uneaten food from the water after 5 minutes. Under normal circumstances your tank only has to be tested for nitrite twice monthly. If your filter stops working test for ammonia within 24 hours to make sure that the good bacteria that eliminates the wastes are still unaffected. Whenever one of your fish seems to be ill you must test immediately for nitrite, simply to rule out nitrite poisoning. Your tank must be cleaned and changed at least once a week or if the water becomes cloudy.

It is cause brown blood disease due to the increase in methemoglobin that causes the blood to take on a brownish hue. This methemoglobin reduces the ability for the blood to carry oxygen around it’s body and in due course will cause suffocation.

Fish contracting nitrite poisoning are affected differently, some species not showing any of the symptoms will just die suddenly whilst others will become sluggish and lethargic. The species that suffer lethargy may suffer from secondary problems over an extended time being exposed to nitrite. The nitrite weakens the immune system and therefore increases the chance of another disease developing, such as fin rot or bacterial infections. Left untreated damage to the liver, blood cells and gills will occur because levels of methemoglobin increase. All this will result in the poor fish dying of suffocation or another secondary complaint.

When changing the water (once a week or when water gets cloudy) add a quarter of a teaspoon of AQUARIUM SALT (not table salt). This prevents the build up of methemoglobin. In order to treat fish that are suffering from nitrite poisoning, do the following :

– Increase aeration and oxygenate the water.

– Don’t put any new fish into the aquarium until the nitrite levels are zero

– Take daily tests until the nitrite has completely disappeared. Don’t stop these tests until that time!