Pool Rx+ [Pool Rx Plus] is the updated algaecide formula from Pool Rx. Pool Rx+ contains three active ingredients Copper Sulphate Zinc Sulfate and Silver Nitrate adding additional layers of protection to the original Pool Rx formula you have come to trust. The yellow Pool Rx+ spa unit is an EPA Registered NSF/ANSI 50 Certified algaecide supporting 100-400 gallons of water. Pool Rx+ reduces chemical demand improves water quality simplifying spa care for up to 6 months. Re-treatment intervals vary based upon conditions such as bather load and splash out. Replace the Pool Rx+ spa/hot tub unit every time the spa/hot tub is drained and refilled or if algae is coming back or the water looks a little dull even though the water is balanced and has at least 0.5-1.0 ppm of chlorine. Pool Rx+ products can support any size body of water & are ideal for use in chlorine Salt UV or Ozone spa/hot tubs.