Q. What is the nitrogen cycle?

A. The first step to good aquarium water quality is to understand the nitrogen cycle. The nitrogen cycle refers to the gradual breakdown of highly toxic ammonia, to NO2 gas. Ammonia enters the aquarium system through fish waste and organic decay. Ammonia occurs in two forms in the water column, free ammonia and ammonium. Free ammonia (NH3) is the most dangerous. When it is present in the water it prevents the animals from excreting their own waste, making the retained ammonia highly toxic. Free ammonia can also enter the animal through its delicate tissue membranes. Ammonium (NH4+) is an ionic form of free ammonia and is less toxic; however, the charged ions of ammonium in the water can burn tissue and cause considerable stress to the animals. When there is excess build up of ammonia in the water it interferes with the hemoglobin in the animal’s blood, making it difficult to transport oxygen throughout their circulatory system. Free ammonia and ammonium exist in an equilibrium reaction so that they are in a steady balanced state. As the pH and temperature increase the ammonium switches over to free ammonia. As the pH and temperature decrease the free ammonia is transformed back into ammonium. In the presence of an efficient biological filter ammonia should pose no threat to a marine aquarium. Nitrifying bacteria, nitrosomonas, oxidize the ammonia into nitrite (NO2). Nitrosomonas require oxygen, ammonia, and a substrate to live on in order to survive and multiply. As ammonium is removed the equilibrium with free ammonia shifts and more free ammonia is converted into the less toxic ammonium. Eventually all the ammonia is broken down into Nitrite. As the ammonia level decreases the amount of Nitrite in the water increases or spikes until the ammonia is at a low enough level for nitrobacter to survive. Nitrobacter is the bacteria that oxidizes the nitrite molecule, breaking it down into nitrate (NO3). Nitrate is the least toxic molecule in the cycle, although at high levels it can be lethal. To maintain a safe level of nitrate regular water changes are necessary. Nitrate can also be removed naturally by anaerobic bacteria (bacteria that don’t require oxygen) through denitrification (conversion of nitrate into nitrogen gas (N2). Denitrification can only occur if the proper conditions are provided. A plenum system with a deep sand bed is one way to induce denitrification by providing the anoxic layer needed by the anaerobic bacteria. Live rock can also be helpful in breaking down nitrates. Once the cycling process is complete in a saltwater aquarium there should be no future occurrences of ammonia or nitrite.