PROtech F065001012AK 1 qt Stain Rust & Scale Control is best used for prevention of metallic stains and hard water scaling. This product sequesters (picks up) the material causing stains and scale and holds them in solution. If stains exist, they may be old ones requiring complete cleaning of the walls or stains of an organic nature. Dissolved metals and minerals (such as iron, copper, calcium, manganese and magnesium) in pools and the corrosion of metal pool fittings will often cause staining and/or scaling. Stain Rust & Scale Control is a diluted solution of hydroxyethylidene-diphosphonic acid (HEDP) that is best used for prevention of metallic stain formation.