Nitrogen is present in water as gas, nitrite, nitrate, and ammonia. Ammonia is the most toxic inorganic nitrogen produced
by fish and by the heterotrophic bacteria of biological filters. EPA considers 0.02 mg/L of un-ionized ammonia as safe (EPA 1991). In recirculating water systems, nitrite toxicity can occur with improper biological filters (Schreck and Moyle 1990). Nitrite is formed in nitrification and denitrification processes and causes methemoglobinemia, and ultimately, hypoxia (Williams and Eddy 1986). Excess ammonia and nitrite levels are primarily responsible for "new tank syndrome" in fish (Moe 1992).