The system should be operating and passing water through the biofilter. If the culture tank is not yet ready for use, it is possible to recirculate water only through the biofilter unit. This situation may arise when starting a newly built facility. System water should be free of residual chlorine that may have been used for disinfection or pathogen control. System water should be warmed or cooled to near the temperature at which the system will be operated when stocked; pH, salinity, alkalinity and hardness should match the requirements of the incoming stock