Algaecide is a chemical treatment designed to kill and prevent algae growth. Rain, debris, and even swimmers can introduce algae to pool water. Pool algae is a harmless form of cyanobacteria, aquatic life that uses photosynthesis to produce an energy and food source. Algae can be free floating, or it can grow in sheets on the walls and floors, and some types of pool algae can survive inside of pipes or filters.

To prevent pool algae, algaecides play an important role. They can be used as an algae-cide to kill small, isolated blooms, but most perform best as an algae-stat, to control and prevent pool algae growth. Pool algaecides are formulated to kill all algae types, but some control certain strains of pool algae better than others, and different algaecides each have their own way of controlling algae.