4-pack of PoolRx Blue Units. Contains 4 individual units. Each PoolRx Blue Unit supports 7,500 - 20,000 gallon pools. Simply place the cartridge in your swimming pools skimmer basket and enjoy an easy to maintain pool. The PoolRx Blue Unit contains a blend of minerals that will continuously and effectively eliminate algae. The specially treated alloy cylinder contains the PoolRx minerals. These minerals immediately dissolve out of the cylinder and form a stable residual throughout the pool water for up to 6 months. These minerals eliminate the algae while freeing up low levels of chlorine to be more active and productive in the water. During the swimming pools normal filtration process the minerals are rejuvenated every time they return and pass through the cylinder. PoolRx works with a constant low level of chlorine. Chlorine Residential levels should be kept between 0.5-1.0ppm; Commercial 1.0-5.5ppm.