1. Large Filter Cartridge for Top Fin: 8 Pack RFL Replace Filter Cartridge. This Filter Cartridge compatible with Top Fin Desktop Aquariums with Large Retreat Filter. Size: 7.48 x 2.67 inches 2. Efficient Water Purification: Our filter cartridge consists of double high density cotton, activated carbon, filter media Zeolite. Double high density cotton can filter impurities and debris from the water. 3 times activated carbon can effectively absorb 99% of the debris and odors. Zeolite further absorbs heavy metals and balances PH. 3. filter medium zeolite: Our zeolite is a reprocessed zeolite. Our spherical zeolite has a stronger cleaning ability, and it can adsorb heavy metals and other harmful ingredients. Balance PH to achieve a stable water state and improve the efficiency of fertilization of aquatic plants 4. Quick Install: Turn off the filter and remove the used cartridge and flush the cartridge under water to remove dust. After rinsing, insert the cartridge into the flushing chamber and make sure the filter cartridge fits completely into the chamber