How do I use shock chlorine?

Shock chlorine is used to quickly raise the free chlorine level when the water needs it. Measure, adjust the pH, then add the dosage on the label, with the filtration system running, without anyone in the pool, and wearing gloves and goggles. Never mix it with another product. Return to the pool when the free chlorine is back within the usual range.

When to shock

The CTX shock chlorine product sheet lists the usual cases: recovering degraded water, after heavy use, after a thunderstorm. It is also used when restarting the pool, as mentioned in our article on tablet dosage.

It is not routine maintenance. If you do it often, a setting needs to be reviewed. Our chlorine maintenance guide details the routine.

Protect yourself and prepare the water

Shock chlorine is a concentrated product. A few steps significantly reduce the risk of accidents:

• read the label and first aid instructions before opening;
• wear gloves, goggles, long sleeves, and closed-toe shoes;
• open the product outdoors, without inhaling the fumes;
• keep children and animals away;
• use a clean and dry dosing cup reserved for this product.

Then measure the pH, TAC, and free chlorine. CTX product sheets require adjusting the pH before adding chlorine. Our rule: if the TAC is below 125 mg/L, or KH below 7, mineralize first, then set the pH between 7.0 and 7.4. One product at a time.

The dosage and procedure

The dosage: only what is on the label

Dosages vary depending on the product. Never apply the dosage of one brand to another.

• CTX shock chlorine, in 1 or 5 kg: 150 g per 10 m³, according to the sheet, with 55% available chlorine.
• Blue Ocean Shock Chlorine: our sheet does not indicate a dosage. Follow the product label.

First calculate your actual volume: accurate dosing starts there. For a round pool, our article on volume calculation will guide you. Do not round up to "go faster": an excess irritates the eyes, nose, and throat.

The procedure

1. Choose the right time. Without anyone in the pool, preferably at the end of the day.
2. Start the filtration and leave it running.
3. Weigh the dosage in the open air, away from the wind.
4. Follow the instructions on the label. If it requires prior dissolution, always pour the product into the water, never the water onto the product: the reverse can cause a violent reaction.
5. Distribute, then rinse the equipment separately.
6. Let it filter for several hours, then measure the pH and free chlorine.

If the test shows an unusually low value right after a shock, be careful: at very high concentrations, the reagent can lose its color. Our article on excess chlorine explains this limitation and how to lower the level.

Should you repeat the shock?

Not out of habit. Water that is still cloudy the next day does not necessarily require a second dose. Measure, clean the filter, and check the filtration time.

CTX shock chlorine is a dichlor: it contains stabilizer, which accumulates with each addition. Too much stabilizer slows down the action of the chlorine, and the problem worsens. Therefore, check this level if shocks are repeated. The CTX stabilizer product sheet indicates maintaining around 25 to 40 mg/L.

Storing the product

Keep the chlorine in its original packaging, tightly closed and upright, in a cool, dry place, away from sunlight, in a ventilated area and out of reach of children. With moisture or prolonged storage, it can degrade and release toxic fumes when opened.

Keep it away from flammable products as well. Return unused leftovers to a hazardous waste collection point, as indicated by your local municipality.

Product references

Frequently asked questions

Can you do several shock treatments?

Yes, if a measurement justifies it, and always according to the label. Wait to know the free chlorine before starting again. If the water remains cloudy despite a shock, first look for the cause: pH, filtration, or stabilizer. Repeated dichlor also increases the stabilizer.

How long to wait before swimming?

There is no single timeframe: follow the label and measure. Return to the usual free chlorine range before getting back in the water. Our guideline for a private pool is 1 to 3 mg/L, with a pH between 7.0 and 7.4.

Can you add shock chlorine and pH minus at the same time?

No. The mixture can release chlorine gas. Pour them separately, with a clean and dry dosing cup for each, and let the filtration distribute the first before adding the second.

What to do in case of splashing or discomfort?

Rinse abundantly with water for at least ten minutes. In case of breathing difficulties, take the person out into the open air. Keep the label handy. In Belgium, call the Poison Center at 070 245 245, or 112 in case of emergency.

What dosage for a 30 m³ pool?

With CTX shock chlorine, the sheet indicates 150 g per 10 m³, or 450 g for 30 m³. This dosage only applies to this product. With another brand, follow only its label.

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For the complete routine: Maintaining your pool with chlorine or salt without the hassle: the complete guide

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If the level remains too high: Too much chlorine in the pool: 6 ways to lower the level, from the gentlest to the fastest

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Last updated: November 4, 2026.