This is a closed looped system with life support. It replicates the natural cycle in which waste products are broken down and the water re-used. Overflow from the tanks is directed to a holding reservoir, pumped through mechanical, chemical, biological and sterilization filters, temperature adjusted, and returned to the animal tanks. Other water parameters can be monitored and/or maintained. 

An additional Supply Water System may be needed (see Basic Reservoir System and Custom Reservoir System) to replenish the unit(s) using the Automatic Reservoir Water Exchange and Re-fill Feature. To prevent Nitrate build-up, a percentage of water in the reservoir is removed and replaced on a daily basis. Water is automatically replaced as it evaporates or is removed from the system. Both the PharmHouse™ ZFS and XLS racks can be used with a re-circulating water system. They can be stand alone units, with the life support located beneath the unit, or linked units, with a larger life support system located nearby. 

Re-circulating Systems are well suited for most fish and crustacean species. They are not recommended for species that are naturally found in very still waters.