By The Zagosa Team. Published .
Reviewed or updated .
Both chlorine shock and non-chlorine shock oxidise organics and break down chloramines. What separates them is the residue. Chlorine shock leaves free chlorine behind, so the tub is out of action for hours. Non-chlorine shock leaves only oxygen and a trace of sulphate, so you can be back in the water within 15 minutes.
When chlorine shock is the right call
After a long spell of disuse (more than two weeks). The free chlorine residual it leaves resets the sanitiser baseline cleanly.
When there is visible biological growth (a green tinge, a slimy waterline). You want the lingering high free chlorine to keep killing as it works through the system.
During an emergency such as a known faecal incident. The dose is far higher (10 to 20 ppm), the wait is at least 30 minutes, and the kill needs to be beyond doubt.
When non-chlorine shock is the right call
Routine weekly upkeep. It knocks down chloramines, lifts oxidation, and lets the tub be used the same evening.
After an ordinary session of two to four bathers. Half a normal weekly dose straight after, cover off for an hour, keeps combined chlorine from building.
In a bromine tub. Non-chlorine shock converts bromide back into active bromine, refreshing the residual without adding any chlorine.
What neither one can do
Neither replaces the routine residual. Free chlorine or bromine still has to be present between sessions. Shock is a top-up, not a substitute.
Neither replaces a refill. Once total dissolved solids climb too high or biofilm takes hold, no amount of shock fixes it.
Neither is a shortcut for low pH. Shock at low pH and you get a localised bleach reaction that can harm the shell. Always balance pH first.
Common questions
Can I use both in the same week?
Yes. Most pros run non-chlorine shock weekly and a chlorine shock once a month or after heavy use. The two work well together.
How soon after a non-chlorine shock can I bathe?
Fifteen to twenty minutes with the jets running, or longer if the bottle says so. The active ingredient, potassium peroxymonosulphate, dissipates very quickly.