Assuming the recirculating system is well managed there are several positive effects of recirculating systems compared to flow through systems. In RAS there is an opportunity to reduce the water consumption, conserve heat (no seasonal interruptions),establish an environmental control that can reduce the risk of disease and pollution problems, and optimize the growth rates and health of the fish by surveillance of the water quality . Another important aspectof RAS is the establishment of microbial control in the cultivation tanks by stabilizing the substrate to bacteria ratio. Variable contents of microbe substrate induces a condition that triggers growth of fast growing opportunistic bacterial species, while stable concentrations of substrate to bacteria induces growth of a slow growing ,stabile and a more beneficent bacterial community referred to as water maturation . Blancheton found that the biofilter was the main source of bacteria in RAS . Depending on operating mode, a conditioned and well functioning biofilter can therefore successively mature and be dominated by a large population of slow growing bacterial types. Under these conditions, nutrients available for growth are consumed there by preventing the opportunistic bacterial populations from growing.