Blue or green stains are a sign of copper: It may have entered your water supply; or, you might have copper pipes that are being corroded. Start by having your water tested for copper; also, check its pH, total dissolved solids content, and oxygen content. Most ordinary water softeners will remove copper, through the ion exchange process. However, if corrosive water is the cause of the problem, you'll need to address that issue, or else the copper will reappear in your water supply. Water with a low pH is considered acid, or corrosive: it attacks metals like copper, causing it to leach into the water. When your water's pH is between five and seven, you can raise it by running the water through calcium carbonate. If the pH is less than five, chemicals must be added. Sometimes, the corrosion may be caused by excess oxygen. To solve this type of problem, a coating substance may be introduced into the pipes, or the water may be aerated, to reduce the amount of oxygen. For more information on preventing blue and green water stains, contact a water conditioning company in your area.