PoolRx 331066 Black Plus Pool Unit Algaecide 20-30K gal contains a proprietary blend of minerals that continuously and effectively eliminate algae. The specially treated alloy cylinder is packed with PoolRx+ granular formulation. These minerals immediately dissolve out of the cylinder and form a stable residual throughout the pool water for up to 6 months. The minerals eliminate the algae while freeing up low levels of chlorine to be more active and productive in the water. During the pool’s filtration process the minerals are rejuvenated every time they return and pass through the cylinder. PoolRx+ works synergistically with a constant low level of chlorine.