HTH Pool Care Shock Advanced Prevents Bacteria & Algae Cal Hypo Formula 1 lbs (Pack of 6) HTH® Pool Care Shock Advanced is the powerful 4-in-1 formula with Cal Hypo that kills and prevents bacteria and algae as well as reduces chlorine odor and irritation for crystal clear healthy water. Traditional chlorine products contain cyanuric acid (CYA). When too much CYA builds up it can cause over-stabilization or chlorine lock and cloud your water. The fast-acting fast-dissolving Cal Hypo formula in HTH® Pool Care Shock Advanced provides chlorination without adding extra CYA for a beautiful sparkle all season. This product is great for all pool types and won t fade your pool liner. How To Use HTH® Shock Advanced: Follow the on-pack instructions according to your pool size to add HTH® Shock Advanced directly to swimming pool water or skimmer. Make sure to use a clean dry scoop. For best results apply HTH® Shock Advanced as part of your weekly pool care routine to keep free available chlorine levels between 1 and 4 ppm.