Water chemistry (Nitrite)

Nitrite
Nitrite (NO2) is formed as part of the nitrogen cycle, when the bacteria in the tank breaks down the ammonia, nitrite are produced. While nitrite isn’t as poisonous to fish as ammonia, it is still very dangerous and can easily kill fish. In a mature aquarium the nitrite levels will always be at zero, thus not being a problem to fish.Nitrite binds to red blood cells and blocks their ability to transport oxygen, because of this affected fish frequently appear to be oxygen-deprived, even in water with high concentrations of oxygen. Affected fish may gasp at the surface or stay near airlines or water inflow. If nitrite is present at lower levels, fish may only show signs of toxicity when they are stressed further and require more oxygen. Over longer periods of exposure to nitrite, fish can become anemic (i.e., deficient in red blood cells).

(please don't mix up Nitrate and Nitrite)