Ammonia is a chemical composed of nitrogen and hydrogen atoms and is most commonly found in aquariums in two forms -- free ammonia and ammonium ion. These two types of ammonia build up in an aquarium through contaminated water, decomposing materials and decomposing fish waste but, due to the ever-changing nature of an aquarium, can never be permanently removed. Ridding an aquarium of ammonia is important, since fish can only tolerate low levels of the chemical.
Stop feeding as soon as you recognise the higher than normal ammonia levels, as excess food contributes to high ammonia levels when it decomposes. Aerate the tank vigorously to knock any decaying matter free from the filter and small spaces.
Complete a partial water change, adding only water that has been properly treated to avoid adding more ammonia. Change the disposable filter elements, then restart the system. Let the tank settle and the filter clean any remaining debris out of the water.
Let the aquarium settle for 48 hours without adding any food. Retest the water for ammonia and repeat Steps 1 and 2 if needed. Monitor the ammonia levels for the next two weeks to ensure the filter is properly coping with the levels of ammonia production.