Algaecide

Algaecide is a supplemental sanitizer, usually a quaternary ammonium compound or polymer, formulated to kill existing algae or make surfaces inhospitable to new growth. It's most useful as a preventive dose after heavy rain or a hot spell, or as an assist during algae cleanup once chlorine levels are already corrected. Algaecide isn't a substitute for maintaining free chlorine and brushing surfaces: used alone, it usually can't keep up with an active bloom, and some copper-based formulas can cause staining if overused.