BioGuard Back Up - 1 Quart, Back Up has a powerful surfactant/wetting agent, which makes it able to work in the tiny cracks and crevices where algae starts, BioGuard Back Up Algaecide is BioGuard's most effective algae preventative, Routine Maintenance: 1 fl oz per 5,000 gallons of water per week, or 2 fl oz per 5,0000 gallons of water every 2 weeks., This highly effective formula prevents wide variety of algae growth