Marina i25 replacement Power Cartridge is made specifically to fit inside the Marina i25 internal filter. Its unique design allows the water to enter through the center of the cartridge. Split into two different chambers, the water first enters the zeolite chamber so toxic ammonia is removed. It then enters the activated carbon chamber so all pollutants and odors get removed. The water lastly enters the floss layer where debris gets trapped inside the cartridge. This unique flow of water results in a crystal clear water. For best results, replace every four weeks.