Ammonia is the primary nitrogen-containing waste product of fish. Under natural conditions, ammonia is converted first to nitrite and then to nitrate. Ammonia and nitrite are toxic to fish at low levels, but nitrate is relatively non-toxic. Ammonia has a variety of toxic effects, including reduced growth rates, whereas nitrite primarily interfers with oxygen transport. The literature pertaining to toxic levels and effects of environmental ammonia and nitrite as well as experimental methods of inhibiting these toxicities are reviewed. Prevention of elevated pH for high ammonia conditions, and the addition of sodium chloride for elevated nitrite levels are suggested as possible inhibitors of these toxicities in channel catfish production.