Once the unit is added to the skimmer (or pump basket), all blue minerals dissolve out of the cylinder and pass through the pool filter and into the water within 4 – 6 hours. The minerals form a stable residual throughout the whole body of water eliminating the algae and bacteria wherever they may be in the pool. These minerals recharge ionically as they pass through the PoolRx cylinder. This is why the minerals are so effective and last up to 6 months, depending on algae kill, backwash and splash-out.