About This Item Low calcium hardness can cause swimming pool water to be corrosive resulting in damage to pool surfaces vinyl linings and equipment. Use HTH Calcium Up to raise the calcium hardness levels in your water and prevent corrosion of your pool surfaces including metal and plastic. HTH Calcium Up is great for all pool types including vinyl lined pools and salt water systems. How to Use HTH Calcium Up Use HTH 6 Way Test Strips or Test Kit to test your water weekly and make sure the calcium hardness level is between 200 and 500 ppm. If it falls below 200 ppm add HTH Calcium Up to raise calcium hardness level. About HTH Pool Care HTH was developed with the pool owner in mind. We know that maintaining your pool water can be a lot of work. That is why we are here every step of the way helping you clear the uncertainty from pool care. With economical easy to use solutions HTH is designed to work hard so you are prepared to tackle whatever life throws in your pool. From balancing your water to treating green water we offer a full line of products to keep your pool water healthy and your swimmers happy.