Aqua Clear 500 Aquarium Power Filter Water input per hour is regulated by adjusting the patented flow adjustment mechanism, tailoring the flow rate to the aquarium's requirements. When the flow adjustment reduces input, the water returned to the aquarium is still equal to input, but the top grid in the filter basket circulates purified water back into the motor chamber so that the pump filters 150 gallons per hour even when adjusted to siphon less. The recommended CycleGuard filter media, when used and rotated as suggested, ensures that both mechanical removal of particulate matter and biological filtration of ammonia and nitrite combine for proven beneficial results to all aquarium inhabitants. CycleGuard inserts break the water stream into many ever-changing channels, passing through the entire length of media before being returned to the aquarium. The CycleGuard Media Step System provides extremely effective mechanical debris removal. The screen size for particles reduces with each step, trapping the large particles first and the fine ones deeper in the chamber. The Step 1 Foam traps larger particulate matter before it can clog and block other inserts. Step 2 Ultragrade Activated Carbon adsorbs liquefied wastes and color bodies and traps fine debris escaping from the foam. CycleGuard is unique--the inserts are designed to be replaced alternately, conserving slowly replicating beneficial bacteria and enhancing the natural reduction of ammonia and nitrite. All inserts provide valuable sites for bacterial colonization. The strong water flow ensures adequate oxygenation through the entire filter chamber volume. Biological filtration is continuous with the CycleGuard system.