Check the listing for details. BioGuard Back Up 2 1qt - 2 Pack. Condition: New. Listed at 57.98 USD. BioGuard Back-Up 2 Algae Inhibitor is specially formulated to work with chlorine in preventing and killing all types of algae. BioGuard BackUp's residual effectiveness stays in the water until it is needed and tends to stabilize the total chemical treatment. It is also the integral third step in the "3 Step System" from BioGuard. Product Benefits: 40% active ingredient Powerful wetting agent Prevents all types of algae growth Works with chlorine Does not evaporate Works in all pH ranges Activity increases as water temperature rises Concentrates on all surfaces No mixing required Non-staining; non-corrosive Dosage: With visible algae - 13.5 oz. per 10,000 gallons No visible algae - 6 oz. per 10,000 gallons Maintenance - 1.5 oz. per 10,000 gallons weekly Distribute directly into the pool water