Nitrogen Compounds & Phosphate Various nitrogen compounds formed in the aquarium are generated from biochemical processes including the breakdown of proteins and waste products from aquatic animals. The principal nitrogen compounds of concern to the aquarium hobbyists are ammonia, nitrite, and nitrate.

Ammonia Ammonia is the most toxic product formed in water. It originates from the decomposition of nitrogen-containing organics such as plants and food. Sources of ammonia in the aquarium water are the fish, other organisms, and decaying food. Ammonia exists in two chemical forms in water: a unionised form (NH3) and an ionised form (NH4+). The combination of these two forms is termed total ammonia. Both exist in water, but the proportion of each type is dependent on the pH, temperature, and other factors. The unionised form is extremely toxic to fish. The higher the pH of the water, the higher the concentration of toxic unionised ammonia. Chronic sublethal concentrations of ammonia in aquariums indicate that there is a serious problem that can be related to various factors, including overcrowding, filter malfunction, or overfeeding. Fully functional and properly conditioned aquariums should have no detectable concentrations of ammonia. Ammonia can easily be detected using commercially available test kits. Some kits will express the result as ion, while others will express the result as ammonia-nitrogen. As a general recommendation, the unionised form of ammonia must not exceed 0.01 mg/L in aquariums. Always follow the manufacturer's instructions exactly to obtain the correct readings for ammonia, regardless of the method for expressing the results.

Nitrite During the development of nitrifying bacteria in the filter bed, the bacteria will transform ammonia into another form of nitrogen called nitrite. Nitrite is the intermediate step in the nitrogen cycle in the conversion of ammonia to nitrate. The highest concentrations occur during the initial establishment of the filter bed. Once the filter bed is established, it is often impossible to detect any nitrite. Though nitrite is less toxic than ammonia, it is still somewhat toxic to water animals because it binds with blood cells, which prevents the normal uptake of dissolved oxygen. The concentration of nitrite is also determined with a test kit. The concentration should not exceed 0.1 mg/L (parts per million, or ppm) as nitrite ion. Continual high concentrations during the initial conditioning period indicate that the nitrogen cycle is not complete.

Nitrate Nitrate is formed from the chemical conversion of nitrite in the aquarium. It is far less toxic than nitrite and considerably less toxic than ammonia. Although it has been thought that nitrate had little effect on aquatic organisms, recent research has shown that high concentrations of nitrate can adversely affect the growth and survival of water animals in the long term. It is recommended that nitrate not exceed 50 mg/L (ppm), and preferably should not exceed 20 mg/L (ppm). The lower limit is often impossible to achieve in some aquariums. Nitrate levels can be tested with commercially available test kits.

Phosphate Phosphate, like nitrate, accumulates in aquarium water over time. Phosphates can become a problem if allowed to exceed allowable concentrations. Excessive phosphate concentrations can affect the health of invertebrates as well as contribute to the undesirable growth of algae. The excessive algae growth on rock in reef aquariums will prevent the desirable coralline algae from developing the concentration of phosphate should be monitored in aquariums. The testing of phosphate in reef aquariums is essential as part of the routine water quality monitoring. It is important to note that phosphate occurs in both inorganic and organic forms. Many test kits only measure the inorganic form, also known as orthophosphate. Therefore, you can make an erroneous assumption that no phosphate is present if you are using a test kit that measures only orthophosphate. You should check with your pet store concerning the correct type of phosphate kit to purchase. The source of phosphate originates from three primary sources: utilisation of food by the aquarium animals, decay of excessive food, and activated carbon and municipal water. You must be careful about not overfeeding, as this contributes to the build-up of phosphate. Make sure that whenever you purchase carbons they are phosphate-free. Most premium brands on the market will note on the label that the product does not contain phosphate. Tap water should also be tested to ascertain if phosphate is present. If your tap water always contains phosphate, you should consider using deionized water for your aquarium. Phosphate levels should generally not exceed 0.2 mg/L for aquariums containing fish.