About this item. The amazing all in 1 weekly formula in HTH Shock Ultra increases chlorine levels kills and prevents bacteria and algae reduces eye and skin irritation prevents stains from metals prevents scale improves filtration and softens water all with results in 24 hours. Traditional chlorine products contain cyanuric acid. When too much CYA builds up it can cause overstabilization or chlorine lock cloud your water and cause algae growth. The fast acting fast dissolving Cal Hypo based formula in HTH Shock Ultra provides chlorination without adding extra CYA for a beautiful sparkle all season. It is ideal for all pool surfaces and will not fade your liner. How To Use HTH Shock Ultra Follow the on pack instructions according to your pool size to add HTH Shock Ultra directly to swimming pool water or skimmer. The built in Visible Blue Action lets you know it is working! For best results apply HTH Shock Ultra as part of your weekly pool care routine to keep free available chlorine levels between 1 and 4ppm. 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.