Robelle Calcium Plus is used to raise calcium hardness levels when measures drop below the recommended range of 200-400 ppm. If calcium hardness levels are too low the "soft water" will corrode pool equipment, etch pool plaster and may cause metallic stains. This is a dry granular product that increases the total hardness in pool water to the recommended range. Directions for use: Test for calcium hardness with a reliable test kit.The desired range for calcium hardness is as follows: Vinyl Pools - 200-300 ppm. Tile and Exposed Aggregate Pools - 200-400 ppm. Marcite, Fiberglass or Painted Pools - 250-400 ppm. If the calcium hardness tests less than the desired range, add this product according to the guide below. Place this product into a plastic bucket already filled with swimming pool water. Do this in a ration of up to 2 pounds of product per approximate 5-gallon bucket of water. Slowly pour this solution around the perimeter of the swimming pool while the circulating system is operating. In a 10,000 gallon pool, do not add more than 10 pounds of this product per 2 hour period.