Water flow is the mechanism by which oxygen is transported into a fish culture vessel and the waste products being generated within are removed. The design of a recirculating aquaculture system (RAS) should ensure that the important parameters affecting water quality and fish productivity, e.g., oxygen, ammonia, carbon dioxide, and suspended solids are properly balanced. This requires calculating the value of each of these parameters independently to determine the thresholds for each. Then, having done the necessary calculations, the system must be operated at the lowest flow rate possible while still maintaining a particular parameter at its design value, e.g., ammonia. Obviously, the minimum flow rate possible while maintaining one particular parameter may be too low for maintaining another. The same mass balance approach can be utilized on any variable affecting water quality. It simply comes down to balancing the transport in, the production of a particular parameter within the culture tank, and the transport out of that parameter (let’s call it parameter ‘x’). In word equation form, engineers like to say:
Transport in of "x" + production of "x" = transport out of "x"
The production or P term can be the production of oxygen, ammonia, suspended solids, or CO2. Note that the production term can be negative, meaning consumption of a certain component, e.g., oxygen. More details can be found in Timmons and Vinci, 2022 Recirculating Aquaculture 5th Edition, Ithaca Publishing Company.