Shocking a Hot Tub

There are two products called shock and they do different jobs. Non-chlorine shock (potassium monopersulfate, or MPS) oxidizes the organic load a soak leaves behind. Chlorine shock (usually dichlor at a raised dose) sanitizes. Reaching for the first when you need the second is one of the most common reasons a cloudy tub stays cloudy.

Which one, and when

The chloramine test

A strong chlorine smell means too little effective chlorine, not too much. What you are smelling is combined chlorine — chloramines — which are the spent product of chlorine reacting with organics. A test strip that separates free from total chlorine will show it: if total reads well above free, that gap is your chloramine load.

The fix is to shock, which oxidizes the chloramines and lets free chlorine come back. Adding more dichlor without shocking simply produces more chloramines. This is the single most useful piece of chemistry on this page, and it is the same reason indoor pools smell the way they do.

Why bromine tubs get more out of it

On a bromine tub, MPS does two jobs at once. It oxidizes the organic load, and it converts spent bromide back into active bromine, which means a weekly shock partially recharges your sanitizer for free. This is a real advantage of bromine and it is why bromine owners can shock routinely without any chemistry penalty. The wider comparison is on bromine vs chlorine.

How to shock, step by step

1. Test first. The reading decides which shock you need.
2. Check pH is in the 7.2 to 7.8 range^[source] before shocking. Shocking badly balanced water wastes product.
3. Take the cover off. Shock needs to off-gas, and a closed cover traps what comes off against the vinyl.
4. Turn the jets on so the dose circulates rather than sitting in one place.
5. Measure the dose for your actual volume, from the product label.
6. Add it across the water surface, not in one spot.
7. Leave the jets running and the cover off for the time the label states.
8. Retest before anyone gets in.

The cover-off step is not optional and it is the one people skip. Shock by-products off-gassing under a closed cover attack the underside of the vinyl, and a cover degrading from beneath is a genuinely expensive consequence of a five-minute shortcut.

How long to wait before getting in

Follow the label, and test rather than timing. Non-chlorine shock typically states a short wait, often around fifteen minutes. A chlorine shock takes longer, because you have to wait for free chlorine to come back down into the normal range rather than the elevated shock level. Testing is the only reliable answer here; the clock is a proxy.

How often

Weekly is the sensible default, plus after any heavy use. A tub used by four people on a Saturday has taken on far more organic load than one used by one person twice, and the shock schedule should follow the bather load rather than the calendar.

If you find yourself shocking constantly and the water still will not stay clear, the shock is not the problem. Either the filter is loaded and cannot remove what the shock has broken down, or the water has reached the end of its service life. Both are covered on how often to change hot tub water.

Safety

Never mix shock products, dry or in solution. Add shock to water, never water to shock. Store containers dry, closed and separated from other chemicals. And remember what shocking is ultimately in service of: maintaining an effective sanitizer residual, which is the core of the CDC’s guidance on controlling Legionella in hot tubs.^[source]