The water used to fill your spa may contain high amounts of metal that can stain or discolor your spa's surface. The presence of minerals can cause oxidizing and scaling on the shell's surface and erosion of the heater element. Serena Spa® Stain & Scale Preventer protects your spa from metal and mineral impurities found in spa water by suspending and inhibiting stain and scale causing minerals. On start-up or initial fill, add 16oz. for spas up to 500 gallons. When you add make-up water to your spa you should add a maintenance dose of 1 oz. per 100 gallons of water.