THE NITROGEN CYCLE

Ammonia, nitrite, and nitrate are intricately related in what is called the nitrogen cycle. An understanding and a mastery of the nitrogen cycle are probably the key elements of water–quality management. The nitrogen cycle refers to the process through which beneficial bacteria reduce fish wastes and excretions into environmentally harmless compounds. First they reduce ammonia into nitrite then nitrite into nitrite. Plants then use the nitrate as fertilizer, or the hobbyist reduces it through regular water changes.
Ammonia (NH3) is the primary waste product of fish, and the initial fuel of the nitrogen cycle. Vented waste makes up 25% of fishes ‘ammonia excretion; the other 75% of the fishes ‘ammonia excretion takes place via osmosis through the gills. Ammonia is not actively excreted, but leaves the fish because a higher level of ammonia exists in the bloodstream than in the surrounding water. When the water contains high ammonia levels, the ammonia does not leave the fishesEbloodstream and they die of ammonia poisoning.
Ammonia is removed naturally from the environment by beneficial bacteria of the Nitrosomonas species that lives on all underwater aquatic surfaces and in the filter. Nitrosomonas pares off ammonia’s hydrogen ions (H+) and replaces them with Oxygen (O2) molecules, creating a nitrite (NO2) molecule.
The need for oxygen in this reaction is illustrated in the following equation:

NH3 + (O2 required)¨(Nitrosomonas)� �NO2 + 3H +

Both ammonia and nitrite accumulation can be detected by simple water test kits readily available at your local fish store. A cycled tank should always have readings of zero for these two compounds.
As we have seen, the beneficial bacteria Nitrosomonas convert ammonia into nitrite. Nitrites are broken down by another beneficial bacteria, Nictrobacter is extremely sensitive to water quality. It will go on hiatus if the water is too low in dissolved oxygen, too warm, too cold, or if it has been treated with almost any additive, or medication, including salt. When the Nitrobacter stop or slow down, you can expect to see an accumulation of nitrite in the system. In addition, the conversion of nitrite to nitrite is only half as efficient as the conversion of ammonia to nitrite, which can also contribute to nitrite accumulation.
Nitrate is the final product of the nitrogen-reduction cycle. It is at this point in the cycle that reduced nitrogen can return to the food chain by becoming available to plants. Nitrate, in the presence of phosphates, makes a vitally important fertilizer for plant life of all kinds. Perhaps the most relevant plant in this regard is the simple algae. Many a garden pond has been clear all winter, and then in the spring, as the Nitrobacter bacteria “warm upEand begin to reduce the winter’s leafy wastes, the pond becomes pea- soup green. Fish tanks which have completed the nitrification cycle may also turn green at the six-week mark, as the ammonia finishes its path from nitrite to nitrite.
Again, nitrate levels can be determined through simple water test. It is recommended that you keep nitrate levels below 50 PPM, which you can do by regular water changes.