Aqua One Algae Block Effectively controls most algae common to freshwater aquariums. It kills growing algae and prevents algae from recurring. To control Algae add one piece per 20L or one block per 120L. This Block is best used as a preventitive or when algae first appears or after cleaning an algae infested tank. Block will dissolve over 7-14 days. If possible control the amount of light on tank to reduce the growth of algae. Repeat dose only when algae reappears. To prevent oxygen depletion remove heavy algae growths mechanically before treatment and aerate the water after use. Active Constituent 2.75g/kg Diuron.