<p>Aqua One Algae Block 20g.<br /><br />Direction for use:<br />To control algae, add one piece per 20L or one complete block per 120L of water. This block is best used as a preventative or when algae first appears or after cleaning an algae infested aquarium. Blocks will dissolve over 7-14 days depending on water conditions and circulation. If possible, control the amount of sunlight or artificial light to reduce the growth of algae. Repeat only when algae reappears. To prevent oxygen depletion, remove heavy algae growths mechanically before treatment and aerate the water after use.<br /><br /></p>