Applied Biochemists Black Algaetrine is a unique, patented blend of algaecides specially formulated to control black algae and other equally resistant strains. Its powerful formula easily penetrates the thick, waxy coating black algae uses for protection, quickly killing it all the way down to its roots, yet contains only 3.07% active copper to avoid any need for water rebalancing. Best of all, Black Algaetrine is made by Applied Biochemists, a leader in bio-based pool chemicals, allowing you to buy with confidence.