In aquaculture it is often necessary to reduce the concentration of ammonia in the water, because it is toxic for the fish. It is particularly important when re-using water or when trans-porting fish long distances without changing the water, since the fish produce ammonium com-pounds as a metabolic waste product.
In water there is an equilibrium between the concentrations of ammonium ion (NH4+) and ammonia (NH3) (Fig. 9.1):
NH3+ H+⇔ NH4+
This equilibrium depends on pH. The sum of NH3 and NH4+ is known as the total ammonia nitrogen (TAN). Because of the equilibrium between NH3 and NH4+, reducing one of them automatically reduces the other. NH3 is the more toxic substance for fish, and therefore is the substance of interest.