American Hydro Systems Formula 2000- Hard Well Water Formula # 2666. One gallon of Rid O' Rust Formula 2000 will treat 125,000 gallons of well water with 1ppm (parts per million) iron and hardness of 15 grains per gallons or more. It is made for use with hard well water and for irrigation applications only. Case contains 4- 1 gallon containers.