The levels of ammonia and toxicity of ammonia are never constant. These are closely related to your pH levels A few hours after feeding the ammonia level will rise as the nitrogen wastes of the fish are excreted. This can actually be measured. A measurement just before feeding should produce the lowest or no ammonia reading (if everything is working). However, an hour or two after feeding a high ammonia level will be recorded as the fish are now excreting their metabolic waste products. Interestingly a corresponding drop in oxygen levels at this time have also been measured in ponds as Koi consume more oxygen in the metabolism of the food.
The higher the temperature the faster the metabolic rate and the quicker ammonia will be released into the surrounding water. However, there is another dynamic related to ammonia – the toxicity of ammonia (NH3) is not constant during the course of a single day. Ammonia is found in two forms in the pond. The toxicity of ammonia is pH dependent. Ammonia changes from ammonia to ammonium (NH4+) as pH drops i.e. it becomes less toxic. As pH rises (naturally during the day) the ammonium (or a percentage of ammonia) converts back to toxic ammonia.
Whilst very high pH above 9 makes a higher percentage of ammonia more toxic and has certain negative effects on Koi, low pH had other effects on the pond system. For example, the bacteria in the filter are oxygen and pH dependent.