American Hydro Systems Formula 500 # 2656-GL The original Rid O Rust formula. One quart of this Formula will treat 125, 000 gallons of well water with 1ppm (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. 4- 1 gallon bottles per case