The National Geographic Bio Cartridge allows important purification bacteria to colonize the cartridgeAAATMs surface, resulting in cleaner water. For use with PF20, PF30, PF50 Power Filters. Features: Fits all PF power filter sizes Includes: 2 - Bio Cartridge Water Type: Fresh water Dimensions: 2.8 in L x 1.5 in W x 8.6 in H Color: Gray and green Caution: This is not a toy. Keep out of reach of children. To avoid injury, follow the safety instructions in the enclosed instruction guide.