In a (koi) pond the water quality can be determined by measuring different water values. The most important values often mentioned are pH, GH, KH, NO2 and NH3. The latter two, nitrite and ammonia, are both toxic for fish and can become a problem in very low concentrations (> 0,2 mg/l).
Ammonia is often not measured directly but can be calculated for example from the measured ammonium total (NH3 + NH4). The well known drop tests made by JBL and Sera both measure the ammonium total. The amount of ammonia derived from ammonium total is also influenced by the temperature and pH.
Ammonia can also be calculated from the measured ammonia nitrogen (NH3-N). The Hanna HI700 photometer is an example of a device often used by koi enthusiasts to measure the NH3-N. The big advanced over the drop tests is the fact that the color change of the water is electronically measured. This is more accurate than using a color sheet and comparing the color change yourself.
The results of a measurement with the HI700 photometer is the NH3-N value in parts per million (ppm). To calculate the amount of ammonia in mg/l simply multiply this value with 1.214.