Ammonium (NH4+) — or its uncharged form ammonia (NH3) — is a form of nitrogen which aquatic plants can absorb and incorporate into proteins, amino acids, and other molecules. High concentrations of ammonium can enhance the growth of algae and aquatic plants. Bacteria can also convert high ammonium to nitrate (NO3-) in the process of nitrification, which lowers dissolved oxygen.
Ammonia in water is either un-ionized ammonia or the ammonium ion. Typically, the value reported is the sum of both forms and is reported as total ammonia or simply - ammonia. The relative proportion of the two forms present in water is highly affected by pH.
Un-ionized ammonia is the toxic form and predominates when pH is high. Ammonium ion is relatively non-toxic and predominates when pH is low. In general, less than 10% of ammonia is in the toxic form when pH is less than 8.0 pH units. This proportion increases dramatically as pH increases.
The equilibrium between NH3 and NH4+ is also affected by temperature. At any pH, more toxic ammonia is present in warmer water than in cooler water.