Back Up is BioGuard's most effective algae preventative. The effective Formula prevents a wide variety of algea growth. Its effectiveness increases with the water tempreture making it ideal for heated pool or pool with a lot of sun. It is able to work in tiny cracks and creveces were algae start. It does not evaporte, effect pH and is non staining. 1 bottle for every 50000L or part there of as a monthly treatment