REPLACEMENT FILTER MEDIA: This Bio-Sponge is specifically designed for the Cascade 600 Fully Submersible Internal Filter (CIF3), and serves as a replacement for when it’s time to cycle out filter media. PHYSICAL & BIOLOGICAL FILTRATION: This replacement Bio-Sponge optimizes the colonization of anaerobic bacteria, along with providing mechanical filtration that traps large particulate matter. 1-PACK: The package only contains a single Bio-Sponge. Although they are disposable, they can be reused time and time again. Simply clean the Bio-Sponge in a separate container within some of your aquarium’s water. This allows for the colony of anaerobic bacteria to remain intact within the Bio-Sponge. MEASUREMENTS: 2.5” (L) x 3” (D) x 5.5” (H) SHOP PENN-PLAX FOR ALL YOUR PET’S NEEDS: Penn-Plax is a manufacturer and distributor of top quality pet supplies, who specialize in creating excellent pet products for animals, both great and small.