It is the core equipment of the whole water treatment process. In the case that the water volume is fixed, the nitrogen input is constantly increasing, and these nitrogenous substances would convert into ammonia or nitrite dissolved in water under the action of organisms. The continuous accumulation of these substances will cause the aquatic organisms poisoning. Therefore, these nitrogenous substances must be converted into non-toxic substances. Biofiltration is the decomposition of ammonia nitrogen and other substances in water by the cultivation of biochemical bacteria through the filtration media, but pre-treatment is required, such as sand filter, protein skimmer can be used as the pretreatment equipment.