A shock treatment (or superchlorination) means raising free chlorine well above normal for a few hours to destroy what maintenance chlorine can't reach: algae, chloramines and built-up organic matter. Afterwards, chlorine drops back on its own to its usual 0.5-2.0 ppm range.
A shock done properly, in order
When you need it
- Green water or algae on walls and floor.
- Strong chlorine smell, red eyes or itching: it's almost never too much chlorine, but combined chlorine (chloramines) above 0.6 ppm. Shocking breaks them down.
- Season opening, after months with the pool out of use.
- After very heavy use (a party, lots of bathers), a storm or a Saharan dust episode.
It isn't a weekly "just in case" treatment. If free chlorine and pH are where they should be and the water looks right, you don't need it.
How much chlorine to add
To get rid of chloramines you need to reach the so-called breakpoint: a practical rule is to raise free chlorine to about 10 times the measured combined chlorine. Algae need a stronger shock and sometimes a second one.
The amount of product depends on two things: the water volume and the strength of the product (90 % chlorine is not the same as 15 %). So:
- Work out the m³ properly with the volume calculator.
- Use the "shock" dose on the label for that volume.
- If you're choosing between products, see what the chlorine percentage means.
Step by step
- Adjust the pH first and bring it to the low end of the recommended range, around 7.2: with high pH, chlorine works far less well.
- Do it at dusk. Sunlight destroys free chlorine; at night the shock works for hours.
- With the pump running, spread the product around the whole pool. If it's granular, dissolve it first in a bucket of water, always adding the product to the water, never the other way round.
- Brush the walls and floor, especially if there are algae.
- Filter continuously for at least 24 hours.
- Test the next day. If the water was green, backwash once it clears.
When you can swim again
When free chlorine is back down to 0.5-2.0 ppm and pH is in range. It usually takes from a few hours to a day, depending on the dose and the sun. In pools for collective use, with free chlorine above 5 mg/l the pool must be closed (RD 742/2013).
If you have a salt chlorinator
Many chlorinators have a superchlorination or boost mode, useful for a small top-up. But when the water is green it falls short: the usual approach is to shock with product and let the chlorinator hold the level afterwards.
Frequently asked questions
How often should I shock my pool?
Only when needed: green water, a strong chlorine smell, season opening or after very heavy use. If chlorine and pH stay in range, there's no need to do it routinely.
I shocked the pool and the water turned green or brown. Why?
If the colour change is immediate and the water doesn't smell of algae, it's usually metal (copper or iron) that the chlorine has oxidised. We explain it in the guide on green water after a shock.
Why do it in the evening?
Because sunlight destroys free chlorine. Doing it at dusk gives the chlorine the whole night to work before the sun starts using it up.