Feature(s): For glass or acrylic aquariums up to 175 gal. Flow rate 800 GPH. 9 3/8" wide 1" bulkhead outflow A built in overflow is the most efficient method of draining aquarium water to a filter. If your aquarium does not have built-in overflow, CPR Continuous Siphon Overflows/Prefilters are the best means for getting water from an aquarium to a filter system without expensive modifications to the tank. They quietly skim up to six times more water than units with tubes. The advantages of CPR Continuous Siphon Overflows/Prefilters include: Adjustable water level allows you to easily change the level in your tank. Black top reduces algae growth. Quiet operation. Tip: We recomond the use of an Aqua Lifter pump with the CPR overflows for trouble free operation. Another alternative is to hook the bleed hose from the overflow to the air intake of a power head. This way the overflow will continue to wok if it drawes some air into it or in the event of a power failure it will restart.