Stain Free is an all-natural stain remover. It’s like Vitamin C for your pool! Stain free is an ascorbic acid product, similar to everyday Vitamin C. It is ideal for treating staining to vinyl pool liners and fiberglass pool surfaces. A safe and people friendly alternative to harsh chemicals typically used to treat stains. The bonus is that Stain free does not create any residual phosphates in your pool water, making it the perfect stain-fighter