FROG Serene Bromine Cartridges (formerly Spa FROG) are for use in the FROG Serene Floating Systems, in which pre-filled mineral and bromine cartridges combine in a floating dispenser to give any spa or hot tub crystal clear water with a silky feel. FROG Serene Mineral Cartridges snap easily into a reusable floating holder and when used in combination with the FROG bromine cartridge, reduces the need for bromine by up to 50%. FROG Serene Mineral Cartridges last an average of four months.