Applied Biochemist 406303A Black Algaetrine Algaecide, 32-Ounce Algaecides is a combination of specially formulated for black algae Contains copper and exclusive penetrating agents to kill and prevent the growth of even the most deeply-rooted algae cells Allows for swimming without waiting; safe for all pool types and will not discolor the water when used as directed Most effective when applied in the morning on warm and sunny days Compatible with chlorine, bromine and ozone sanitizers