Nitrifying bacteria are a type of bacteria that live in water and convert ammonia into nitrite. There are two types of nitrifying bacteria, one that converts ammonia directly into nitrite and the other that converts ammonium ions into nitrate. These two types of bacteria typically coexist in natural waters. Some aquarists use them to keep the aquarium clean and healthy while others use them to keep their fish healthy by removing harmful chemicals. However, adding too much nitrifying bacteria can create problems as it can cause algae to bloom or kill off other species of beneficial bacteria.