Ammonium and ammonia are mainly produced from fish droppings. Depending on the pH value, ammonium changes into ammonia and vice versa. While ammonium is largely harmless, ammonia is toxic to fish.
With the sera NH4/NH3 test it is possible to determine both values (with the help of the pH value, you can read the actual ammonia value in the table accompanying the test)
It is enough for about 60 measurements.