Eliminates phosphates (algae's primary food source) to starve algae. Can be used all season & is perfect for pool closing in the fall to control algae during the winter months. Unlike algaecides that kill existing algae (resulting in a release of phosphates by the dying algae), phosfree eliminates phosphates from the water to solve even the most stubborn algae problems. Weekly dosage: 4 ozper 8, 000 gallon, (increase dosage for excessive phosphate levels).