The major source of ammonia in a water of a heavily stocked culture pond or in the effiuent of a raceway is from excretion of fish, mostly via their gills. Ammonia is produced by animals as a byproduct of protein metabolism. What is measured by chemical analysis (Nessler method)for ammonia is called total ammonia nitrogen(TAN)because it includes two forms of ammonia:ammonia (NH3), the unionized form,and the ammonium ion(NH4+). The unionized ammonia(UIA)is toxic to fish