Toxic levels, how it works:
Nitrite notably oxidises the iron in their haemoglobin to produce a molecule called methaemoglobin, which unlike haemoglobin, can't carry oxygen around the blood. This process (called methaemoglobinemia) sometimes turns the blood brown and causes extreme breathing difficulty or even suffocation.
Nitrite also builds up in the blood causing poisoning, and may lead to liver, gill and blood cell damage.
Exposure to nitrite for long periods makes the immune system less effective and the fish may start to suffer from diseases, such as white spot and bacterial infections, like fin rot or ulcers, if they haven't already died from nitrite poisoning.
The addition of salt provides chloride which reduces both the methaemoglobinemia and the toxicity of nitrite in the blood. However, interestingly, although the chloride has always been provided via sodium chloride, more recent research suggests that calcium chloride can work as well, if not better, because the additional calcium is also thought to decrease gill permeability, which prevents as much nitrite entering the blood.
For this reason, fishes kept in water with a higher calcium and chloride level, are usually less sensitive to nitrite than those in freshwater with a low chloride and calcium concentration - but many marines remain sensitive.
Nitrite notably oxidises the iron in their haemoglobin to produce a molecule called methaemoglobin, which unlike haemoglobin, can't carry oxygen around the blood. This process (called methaemoglobinemia) sometimes turns the blood brown and causes extreme breathing difficulty or even suffocation.
Nitrite also builds up in the blood causing poisoning, and may lead to liver, gill and blood cell damage.
Exposure to nitrite for long periods makes the immune system less effective and the fish may start to suffer from diseases, such as white spot and bacterial infections, like fin rot or ulcers, if they haven't already died from nitrite poisoning.
The addition of salt provides chloride which reduces both the methaemoglobinemia and the toxicity of nitrite in the blood. However, interestingly, although the chloride has always been provided via sodium chloride, more recent research suggests that calcium chloride can work as well, if not better, because the additional calcium is also thought to decrease gill permeability, which prevents as much nitrite entering the blood.
For this reason, fishes kept in water with a higher calcium and chloride level, are usually less sensitive to nitrite than those in freshwater with a low chloride and calcium concentration - but many marines remain sensitive.
