The recirculating aquaculture system (RAS) is based on the fact that the water can be used several times when it is purified in the system.
The ammonium fish excretes is very toxic and it accumulates in the system, but with biofilters it can be turned into less harmful forms. The process is called nitrification. The nitrifying bacteria transform ammonium into nitrites and further nitrate, which is not that toxic. This process is aerobic. To tolerate the accumulating nitrate levels the water can either be changed in appropriate flowrate or use another biological process, denitrification.
In denitrification process the bacteria transform nitrate into nitrogen gas, which can be released in air. Thus the amount of water needed for production can be really small, only around 1 % compared to the amount needed in traditional flow-through systems.
The system components and sizing depends on the fish species, the water source and temperature etc., so every RAS system is designed according the demand.