What is recirculating in aquaculture/Pisciculture systems?

This is a method by which exchange of water is minimized due to use of bio-filtration and other environmental control techniques to recycle the used water. The water is treated with a series of processes such that the need for fresh, clean water is reduced to maintain fish or other aquatic stock in the farms. Some of the measures in the recirculating of water involve bio-filtration, solid removal, oxygenation, pH control, temperature control and bio-security.