SpaChoice 472-3-5031-02 Increase Calcium for Spas and Hot Tubs (2 Pack), 14 oz Increases Calcium Hardness in Spa Water Low hardness will etch plaster and corrode spa equipment The ideal hardness level is 200-400 ppm Use 1 oz. per 500 gallons to increase hardness by 10 ppm Use a test kit or test strip to test your spa calcium levels Downloadable User Guide Available Under Product Information: Technical Specifications