BioGuard's most effective algae preventative. The highly effective formula prevents wide variety of algae growth. It's stable in sunlight and will not evaporate. The activity of this product increases as water temperature rises, which is when you need protection the most. Back Up has a powerful surfactant/wetting agent, which makes it able to work in the tiny cracks and crevices where algae starts. This product will not affect pH levels and is non-staining.