Nitrifying bacteria are a kind of autotrophic bacteria that can degrade ammonia and nitrite, including nitrifying bacteria and nitrifying bacteria, belonging to an independent family-nitrifying bacteria. Nitrifying bacteria can obtain energy by oxidizing inorganic compounds through nitrification to meet their own metabolic needs, and take CO2 as the only carbon source, which is a typical chemotactic inorganic bacteria. Nitrifying bacteria have nitrification, which refers to the process that nitrifying bacteria oxidize NH3 to NO- and further oxidize it to NO-3 under aerobic conditions, and obtain the energy needed for growth. Nitrification can be divided into two relatively independent and closely related stages. In the previous stage, NH3 was oxidized to NO-2, which was called nitrosation or ammonia oxidation, and was completed by nitrosobacteria. In the later stage, NO-2 is oxidized to NO-3, which is called nitrification, which is completed by nitrifying bacteria. So nitrification, which is often mentioned, actually includes two stages: nitrosation completed by nitrosobacteria and nitrification completed by nitrifying bacteria. Nitrifying bacteria are indispensable members of aquaculture water ecosystem and have important application value in aquaculture. The application in aquaculture is the application of nitrifying bacteria in ammonia nitrogen oxidation and nitrite oxidation. Among the three nitrites (NH3-N, NO2-N and NO3-N) in aquaculture water, NH3-N and NO2-N are more toxic, and nitrifying bacteria can transform nitrite into nitrate, which can be used by algae to purify water. At the same time, nitrifying bacteria can assimilate and dissimilate hydrogen sulfide while synthesizing their own substances, thus achieving the functions of purifying water quality, improving pond sediment and maintaining a good aquaculture ecological environment. However, the long growth time of nitrifying bacteria limits its wide application in aquaculture.