The pre-filled chlorine Bac Pac is inserted into the Mineral Reservoir and adds low level chlorine support for 1-3 weeks, which works along with the minerals to completely eliminate bacteria and allows you to cut chlorine use by up to 80% over standard water treatment programs. For use only with the Pool Frog® Mineral Inground Pool System or the Pool Frog® Mineral Above-ground Pool System. Due to federal regulations, Bac Pacs can only be shipped in 12 Packs.