The nitrogen cycle is a common biological process that occurs innature, and it also occurs in our aquariums. In aquariums it’s an essential process because it removes toxic wasteproducts and coverts them to a much less toxic form. The process is performed by bacteria, whichwe call beneficial bacteria (or BB). These beneficial bacteria colonize any surface they can in the aquariumand in the filters. Special types offilter media, called biological media, are designed with as much surface areaas possible to provide good places for these bacteria to colonize.
Ammonia is produced by fish waste, fish respiration, and uneaten fishfood. It’s very toxic to fish, andbecomes even more toxic at a higher pH. Ammonia toxicity causes symptoms like prominent red veins in fins, redsplotches on the body, and gasping at the surface. It’s converted into nitrite by beneficialbacteria called nitrosomonas, so in a cycled aquarium, ammonia should always bezero.
These nitrosomonas produce nitrite. Nitrite is also very toxic to fish, causing symptoms like gasping at thesurface, lethargy, hanging near the surface, and tan or brown gills. It impedes the ability of the fish’s blood tocarry oxygen. Nitrite is converted intonitrate by beneficial bacteria called nitrobacter, so nitrite should always bezero in a cycled aquarium.
Bacteria called nitrobacter produce nitrate. Nitrate is much less toxic to fish, but isstill harmful in great quantities. Nitrate toxicity can cause loss of proper swim bladder function andlethargy. In a cycled aquarium, nitratewill continue to accumulate. Live plantsconsume some nitrate, but mostly it is removed by regular water changes. You should typically not allow the nitratesto go above 20 ppm, but different species of fish have different ranges ofnitrate tolerance.