Penn-Plax presents the Cascade 600 Filter Replacement Bio-Sponge. 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. The Bio-Sponge aids with keeping the water inside your tank crystal clean, allowing for a safe and healthy environment. 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 with 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.