Ammonia – When the fish eat food, they produce ammonia as waste. At high levels, it will burn the gills of the fish and can cause death. Even at low levels, it can create serious health problems. If the gills get burned, it reduces the ability for the fish to excrete ammonia through the gills, thus causing the ammonia to build up inside the fish quickly. Even low levels of ammonia in a pond for an extended period will cause stress in the fish, thereby lowering its ability to fight off bacterial infections and parasites. For this reason, the only acceptable ammonia level in a pond should always be 0.
What can cause a high ammonia level?
Filtration – The pond filter may be inadequate for the pond by either being too small for the fish load or just a poor design. In the case of a new pond, the filter may not have had enough time for the bacteria to develop. In cold climates, the filter bacteria will die back in the wintertime. If you feed the fish too much food in the spring before the filter bacteria have a chance to build back up, you may experience high ammonia levels. Giving the fish a diet high in protein early in the spring or late in the fall can create ammonia because the filter is not up to speed. If you add too many fish to the pond at one time, you can experience an ammonia spike. If you medicate the pond with formalin, potassium permanganate, or several other medications and do not bypass the filter media during the process, you can kill off some of the bacteria in the filter, allowing an ammonia spike. Additionally, if the alkalinity (concentration of bicarbonates) in the water is low (below 50 ppm), this can significantly reduce the ability of the nitrifying bacteria to form.