he major excretory product of fish and many other aquatic organisms. Ammonia is formed during the utilization of proteins which the organism receives through its diet. In fish, the majority of ammonia is excreted through the gills.

Ammonia exists in two forms in water, the gaseous or un-ionized form (NH3) and the ionic form (NH4+) which is called ammonium. While the sum of the two forms (called total ammonia) always equals 100%, the percentage present in the water of either form depends mostly on the pH of the water and, to a lesser extent, the water temperature and salinity. For example, assuming a total ammonia concentration of 1 mg/L at a pH of 7.5 and a temperature of 75°F (24°C), the gaseous fraction (NH3) will be 0.0165 mg/L, while the ammonium concentration is 0.9835 mg/L, with the sum of total ammonia equaling 1.0 mg/L. If everything is kept the same except the pH is increased to 8.5, the NH3 value becomes 0.144 mg/L while NH4+ will equal 0.856 mg/L (the sum still equaling 1.0 mg/L).

Ammonia is acutely toxic to fish at concentrations near 1 mg/L of NH3. Chronic low values of ammonia (e.g., 0.1 mg/L NH3) result in stunted fish growth and increased stress which can lead to secondary bacterial infections. The ammonia level in an aquarium should be maintained as close to 0.0 mg/L as possible.