Tetra ReptoFilter Cartridge refills work effectively to keep water crystal clear and remove odors from terrarium water. These disposable filter cartridges are tough on debris – Ultra-Activated carbon absorbs odors and discoloration, and dense, dual-sided mesh catches debris and waste. They’re convenient, fully assembled and ready to use. Use these medium-sized filters with Tetra ReptoFilter 20 gallon, Viquarium™ and Decorative ReptoFilter. Replacing cartridges is fast and easy. Rinse the cartridge under cold water to remove carbon dust, then remove the Tetra® ReptoFilter lid and insert the cartridge into “V” slots and slide it all the way down into the filter. Replace monthly, or when water begins to backflow over the top of the clogged cartridge.