From The Aquarium Wiki
What is it?
Nitrite poisoning (often called brown blood disease as it turns the gills of fish from a bright red to a dark brown-like colour) is caused by having too high a level of nitrite in your tank water. In a correctly cycled and maintained tank, this level will be zero ppm.
Nitrite is a toxin and small levels are deadly to fish and other aquatic animals.
There is no safe level other than zero. Different species of animals have different tolerance levels. Even low concentrations of 0.5mg/l over extended periods can cause long term damage and seriously reduces the ability of the animal to fight off infections.
What causes it?
Adding aquatic animals before the tank is cycled. Placing too many animals in a tank before the volume of nitrifying bacteria in the tank/filter has a change to grow to match the new level of ammonia being produced by the animals. Not enough filtration.
What are the effects?
The nitrite also damages the nervous system, liver, spleen, and kidneys of fish and other aquatic animals. So their immune system is compromised and they can't fight off infections at a later date. Often fish can die days or even a week later.
What to look for
Typical signs are fish, which don't normally supplement their intake of oxygen from the surface, trying to breath faster and will try to get extra oxygen from the oxygen rich layer just below the surface.
Eventually the fish tires and suffocates or is seriously infected by fungus or bacteria at a later date.
- Always have a water test kit to hand that measures nitrite levels. This will confirm your diagnose.
Water changes are the primary fix for reducing nitrite levels. By changing the tank water for clean, you dilute the toxin.
- Stop feeding until the level is zero and then feed very lightly for a week afterwards.
The water conditioner bottles Prime by Seachem or Amquel+ by Kordon claims to reduce or neutralise the nitrite toxin effect within minutes. So adding this will be beneficial. But note these chemicals also briefly reduce the level of oxygen in the water, so ensure good water circulation or aeration if your tank is overcrowded.