The levels of ammonia and toxicity of ammonia are never constant. A few hours after feeding the ammonia level will rise as the nitrogen wastes of the fish are excreted. This can actually be measured. However, an hour or two after feeding a high ammonia level will be recorded as the fish are now excreting their metabolic waste products. Interesting, a corresponding drop in oxygen levels at this time have also been measured in ponds as the koi consume more oxygen for the metabolism of the food.
Ammonia is found in two forms in the pond.
The toxicity of ammonia is pH dependant. Whilst very high pH above 9 makes a higher percentage of ammonia more toxic and has certain negative effects on koi, low pH has other effects on the pond system. The nitrosomonas bacteria that oxidise ammonia to nitrite have a optimum pH range of 8.0 - 8.8. The lower limit for reasonable growth for nitrosomonas bacteria is given as pH 7.0 - 7.6. While the upper limit is 9.4.