Ammonia and nitrite are highly toxic to fishes, with ammonia occurring in surface waters more commonly than nitrite. Nitrate is a related compound but is not significantly toxic to fishes. The acute toxicity of ammonia to aquatic organisms is affected by water pH, dissolved oxygen, temperature, concentration fluctuations, degree of salinity, presence of other chemicals, and prior acclimation. The acute toxicity of nitrite is known to be affected by water pH and the presence of chloride and calcium. More research is needed on the effects of these and other variables on the acute toxicity of both ammonia and nitrite, as well as the chronic effects of both of these toxins.