The Marina i25 Replacement Power Cartridge is made specifically to fit inside the Marina i25 internal filter. The outside of the power cartridge features polyester foam to trap fine particles; while the inside chambers contain carbon to remove pollutants and odors; and zeolite to eliminate toxic ammonia for a cleaner, healthier aquarium environment and crystal clear water. Note: For best results replace every 4 weeks. Key Features: 2-pack Provides mechanical, biological, and chemical filtration Polyester foam traps fine debris; carbon removes pollutants and odors; and zeolite eliminates toxic ammonia Easy to replace Designed to fit the Marina i25 Internal Filter (A131)