Hi-Temp Spa Hardness Increaser is used to increase the calcium hardness of spa water. Spa surfaces may etch if calcium hardness is below 175 ppm. Test the water with a reliable test kit. Acceptable calcium hardness is within a 175-275 ppm range.\n1.5 ounce of HASA HI-TEMP SPA HARDNESS INCREASER in a 500 gallon spa will increase the calcium hardness approximately 10 ppm. Add this product slowly through the spa skimmer with the pump running. Do not clean filter until material has completely dissolved. Re-test spa water after twenty-four hours and reapply as necessary. Do not over treat.\nWater hardness is a concentration of the calcium and magnesium in your spa water. Signs of high hardness are cloudy water; scale formation; skin and eye irritation; and foamy water. Signs of low hardness are corroded metals/equipment; cloudy water; and foamy water.