Marina i25 Internal Filter Refill Cartridge. 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).