Pool RX 99066 Mineral Unit for 20,000–30,000 GallonsKey Function Mineral Based Algae Control Releases proprietary minerals that circulate through the pool to inhibit algae growth Treats 20,00030,000 Gallon Pools Correct sizing for medium to large residential pools using the Black White (Silver) unit 331066 Reduces Chlorine Demand Supplements existing sanitizer systems so you can maintain clean water with less chlorine Long Lasting Performance Provides continuous algae prevention for up to 6 months