Hot tub shock: when and how to do it

Shock your hot tub about once a week, and after any heavy use, by adding a spa shock per the label with the jets running and the cover off for at least 20 minutes. Use non-chlorine shock for routine weekly care and chlorine shock when the water is cloudy, smelly, or has been neglected.

What shock does

Every soak adds sweat, skin oils, lotion, and deodorant to the water. Sanitizer attacks that waste and gets used up doing it. Used-up chlorine becomes combined chlorine (chloramines), which is what causes the sharp "chlorine" smell and red eyes. Shock is a strong oxidizer that burns off that waste and frees your sanitizer to work again.

In a bromine tub, shock has a second job. It turns spent bromide back into active bromine.

Which shock to use

Type	Active ingredient	Best for	Wait before soaking
Non-chlorine shock	Potassium monopersulfate (MPS)	Weekly maintenance in chlorine or bromine tubs	About 15–20 minutes, per the label
Chlorine shock	Sodium dichlor	Cloudy water, odor, after a party, fresh fill, any sign of algae or slime	Until free chlorine is 5 ppm or less

Non-chlorine shock oxidizes but does not disinfect. If the water has a real sanitation problem, use chlorine shock. Dichlor shock is safe to use in bromine tubs too.

When to shock

• Once a week as routine.
• After heavy use, such as several people or a long soak.
• After a refill, once the water is balanced.
• When total chlorine reads more than about 0.5 ppm above free chlorine.
• When the water is cloudy, dull, or smells.
• When the tub has sat unused for a week or more.

How to shock a hot tub

1. Test the water. pH should be 7.2–7.8. Shock works poorly at high pH. Adjust pH first and wait 20 to 30 minutes.
2. Take the cover off. Shock releases gases that need to escape. Trapped under a cover, they damage the cover's underside and pillows.
3. Run the jets on low. Turn the air controls off so the water mixes without foaming.
4. Measure the dose from the label for your tub's volume. Use a clean, dry scoop.
5. Add the shock to the water. Sprinkle it over the center or pre-dissolve it if the label says so. Do not add it through the filter compartment unless the label directs it.
6. Leave the cover off for at least 20 minutes with the jets running, longer for chlorine shock.
7. Retest before soaking. For chlorine shock, wait for free chlorine to come back to 5 ppm or less.

Safety

• Never mix shock with any other chemical, wet or dry. Do not use the same scoop for two products.
• Add shock to water, never water to shock.
• Do not add shock right after pH decreaser or other products. Leave 20 to 30 minutes between additions.
• Store it cool, dry, sealed, and away from other chemicals.
• Keep people out of the tub while shocking.

If shocking does not fix the water

Shock clears oxidizable waste. It does not fix everything. If the water is still off a day later, check these.

• Filter. A dirty filter puts waste straight back into the water. Rinse it, or soak it in filter cleaner. See how to clean a hot tub filter.
• pH and alkalinity. High pH makes chlorine weak and water cloudy.
• Old water. After 3 to 4 months, dissolved solids build up and shock stops helping. Time to drain and refill.
• Biofilm in the plumbing. If sanitizer vanishes overnight or flakes appear when the jets start, do a line flush at the next water change.

How much is too much?

Over-shocking is rarely dangerous if you stay out until levels fall, but it wastes product, bleaches swimwear, wears out pillows and covers, and can read falsely on test strips. Very high chlorine can bleach a strip's pad white so it looks like zero. If you suspect that, wait a few hours with the cover off and test again.

Do I need shock if I have ozone or a mineral system?

Yes, though usually less of it. Ozone and minerals reduce how much sanitizer you need, but they do not leave a lasting residual in the water. You still need a measurable chlorine or bromine level and a periodic shock.