Swim world calcium eliminates corrosion due to soft water in swimming pools. Maintain calcium leavels between 200-400 ppm for vinyl and gunite pools. Test the water with a reliable hardness test strip. 10lbs in 10,000 of water will raise the leavel 80ppm. CAUTION dilute with water before adding to pool. Mixture will get hot, do not stir with hand or metal object. Pour diluted mixture in deepest end of pool. 50lb bag.