Help make your tank more hospitable for underwater buddies TL Reefs Deionization Resin Aquarium Prefilled Cartridge. It works to fully remove all ions and provide zero TDS water with premium-grade， gel type， high capacity， color indication mixed ion exchange resin. This cartridge is specially manufactured with a color indicator to visually show when the resin is exhausted and needs replacing. The resin is also vacuum-sealed in single-use mylar bags to prevent any air intrusion. TL Reefs Deionization Resin Aquarium Prefilled Cartridge can be used for all freshwater and saltwater aquariums and ponds to create ultra-pure water.