This Marina I25 filter cartridge is designed for the Marina I25 internal filter (A131). It features a unique two-chamber design that forces water through a zeolite chamber to remove toxic ammonia, followed by an activated carbon chamber to remove pollutants and odors. The outer layer is made of polyester foam to trap fine particles. This cartridge provides mechanical, biological, and chemical filtration, contributing to a cleaner and healthier aquarium environment. It is recommended to replace the cartridges every 4 weeks for best results. Each pack contains 2 cartridges.