Size:18-count The Marina Slim Filter Carbon Plus Ceramic Cartridge combines activated carbon and Ceramitek in one cartridge for a healthy aquarium. The activated carbon effectively removes pollutants and odors, while the Ceramitek, a highly porous ceramic biological filter medium optimizes biological filtration. The easy-to-use disposable cartridge should be replaced every two weeks. Make sure to never replace all filter cartridges at the same time in order to ensure constant presence of beneficial bacteria. The Marina Slim Filter Carbon Plus Ceramic is best for Tropical fish.