Nitrate is a compound that is not toxic to fish at concentrations typically found in ponds. Problems arise during the cooler months when large amounts of ammonia build up as a result of reduced phytoplankton and bacterial activity. To compound the problem, the bacteria that convert ammonia into nitrite work faster than the bacteria that convert nitrite to non-toxic nitrate in cool water. It should be noted that high concentrations of nitrite can develop in cool water that has elevated levels of ammonia within 24 hours. Prevention and Treatment Nitrite toxicity does not have to be a problem in catfish ponds because it is one of the easiest diseases to prevent.
Chloride competes with nitrite at the fishes' gill surface for absorption.
It is recommended to maintain at least 60 ppm of chloride in water at all times to prevent nitrite induced brown blood. Ponds should be checked for chloride levels several times each year. It is very important to enter the fall with at least 60 ppm chlorides. Monitor chloride levels monthly through the winter and spring to maintain 60 ppm. Make sure to check chloride levels after periods of heavy rain, as rain water will lower chloride levels.