American Hydro Systems Formula 500 # 2656 The Original Rid O' Rust Formula One quart of this Rid O Rust Formula 500 will treat 125,000 gallons of well water with 1 ppm (part per million) iron. It is most effective when the pH of the water is 6.0 or higher and hardness measures 15 gpg (grains per gallon) or less. It is made for irrigation applications only. 12- 1 quart bottles per case