The Seapora Power Filter PF30 is a hang-on-tank power filter designed to provide multi-stage (chemical, biological, and mechanical) filtration through the use of a reusable bio-sponge and disposable carbon/floss cartridge. This filter is powerful and quiet, with simple installation and maintenance and adjustable flow control. Seapora has created this filter for a wide variety of tank types and sizes, featuring four selectable filter sizes for you to choose from to accomodate the size of your tank. The PF30 is a dual-overflow unit which requires the installation of two Small size Seapora Carbon Cartridges as well as two Seapora Bio-Sponges, and two of each are included in the package. Replacement Seapora Bio-Sponges and Seapora Carbon Cartridge refills are sold separately.