Box of 6!
Non-chlorine shock or "Monopersulfate" (also marketed as "Active Oxygen") is a powerful oxidizing agent. It kills bacteria and eliminates residues left behind by bathers: sweat, sunscreen and so on.
Some people rely on Active Oxygen as an alternative to chlorine or bromine. We don't recommend this unless you know that you're allergic to both chlorine and bromine; it's good stuff, but it's so unstable that its anti-bacterial effect doesn't last very long. It is much better to use it as an occasional shock treatment
Shock treatments are essential for maintaining clean water in your spa. You should shock the water once a week or whenever the chlorine or bromine level drops to zero. For heavily-used spas (including any commercially-operated spa), add shock at the end of each day.
The main alternative to non-chlorine shock is... wait for it... chlorine shock. The problem with chlorine shock products is that the really good ones are unstable and dangerous to handle and store. We don't sell them. A safer but weaker alternative is granular "dichlor" chlorine, but you have to add a lot of it, then wait 24-48 hours for the chlorine levels to drop so that the water is safe to use again. The big advantage of non-chlorine shock is that you can add just one or two scoops then use the spa 5 minutes later.
6 x 1.2kg