Ammonia
Ammonia (NH3) is caused by the breaking down of waste products in your tank. This waste includes uneaten food, fish waste, dying plants; and the fish themselves also excrete ammonia out their gills. Ammonia is the first step of the nitrogen cycle, fortunately in a matured aquarium the bacteria will turn this ammonia into less harmful nitrite and nitrate. Ammonia will only occur in the water if there are not enough bacteria to cope with the amount of waste. Ammonia is deadly and will kill fish very quickly.