Penn-Plax presents the Cascade 600 Filter Replacement Carbon Cartridges. These Activated Carbon Cartridges are specially designed for the Cascade 600 Fully Submersible Internal Filter (CIF3), and serve as a replacement for when it's time to cycle out aquarium filter media, keeping the water inside your tank crystal clean, which allows for a safe and healthy environment. Chemical Filtration: In addition to absorbing ammonia, nitrites, nitrates, and phosphates in a timely manner, these Activated Carbon Cartridges also remove harmful chemicals and toxins, clear up discolorations, and reduce foul odors from your tank. Top-Quality Activated Carbon: These Activated Carbon Cartridges come together in a 2 Pack. For best results, replace cartridges every 4 to 6 weeks. With that said, they are refillable, allowing you to customize filtration based on your aquarium's needs. Measurements: 4” (D) x 3” (D) x 3” (H) each. 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.