Short answer
To shock a pool, test the water and lower the pH to about 7.2, then add a large dose of chlorine shock in the evening with the pump running, pouring liquid shock in front of a return. Brush the walls and floor, leave the pump on overnight, and retest before anyone swims.
Shocking is just raising the chlorine high enough, for long enough, to burn off whatever the normal level was losing to. Most of getting it right is timing and pH. Here is when a pool needs it, which shock to use, how to add it safely, and what to expect afterward.
When should you shock a pool?
When the chlorine is losing. The usual times:
- After heavy rain or a run of storms.
- After a party or a lot of swimmers.
- When the water turns cloudy or starts to tint green.
- When free chlorine has dropped close to zero.
- After a power outage that kept the pump off.
In a Florida summer that can be every week or two. In winter it is much less often.
What is the best shock for a pool?
Liquid chlorine (sodium hypochlorite) and granular calcium hypochlorite are the two common chlorine shocks. Liquid mixes in at once and adds no calcium, but it is weaker by the gallon and loses strength sitting in a hot garage. Calcium hypochlorite is stronger by weight, but it adds calcium and can cloud water that is already hard. Nonchlorine shock (potassium monopersulfate) breaks down swimmer waste but does not kill algae, so it is no help on a green pool.
How much shock do I put in my pool?
Follow the label for your pool's gallons, because strength varies from one product to the next. A routine shock is the label's normal dose. A pool with algae needs a lot more, often two or three times that, held high until the water clears. How to get algae out of a pool covers a green pool step by step. Or hand it off: a green pool cleanup does the testing, the shocking and the follow up for you.
Should you shock a pool during the day or at night?
At night. Sunlight breaks down chlorine fast, so shock added at noon loses a lot of its strength before it has done much. Added after sunset, it has the whole night to work.
Can you shock a pool in the rain?
You can, but it is better to wait for the storm to pass. Heavy rain dilutes the dose and can overflow the pool, carrying some of it away. Shock that evening once the rain stops, which is when the pool needs it most anyway.
Can you put shock in the skimmer?
No. Shock poured into the skimmer goes straight through the pump, filter and any heater at full strength, and calcium hypochlorite can react dangerously with chlorine tablets sitting in a skimmer or feeder. Pour liquid shock in front of a return with the pump running, and dissolve granular shock in a bucket of pool water first unless its label says to spread it dry.
How long after shocking can you swim?
Once free chlorine is back to 5 ppm or lower and the pH is in range, which is usually the next morning. The full answer is in how long after shocking a pool you can swim.
Is pool shock dangerous to handle?
It is a strong oxidizer, so treat it with respect. The CDC says to store pool chemicals somewhere cool and dry and to keep different chemicals from mixing. Never put two products in the same bucket, wear gloves and eye protection, and keep everything locked away from children and pets.
How to shock a pool
- 1
Test the water
Check free chlorine and pH so you know where you are starting.
- 2
Lower the pH
Bring the pH to about 7.2 to 7.4. Chlorine works much harder at the low end of the range.
- 3
Wait for evening
Shock after sunset so the sun does not burn it off before it works.
- 4
Add the shock with the pump running
Pour liquid shock in front of a return. Dissolve granular shock in a bucket of pool water first unless the label says otherwise.
- 5
Brush the pool
Brush the walls, steps and floor to stir up anything hiding and spread the chlorine around.
- 6
Run the pump overnight and retest
Leave the pump on all night, then test again in the morning before anyone swims.
Quick answers
- Can you shock a pool two days in a row?
- Yes, if it needs it. A green pool often needs the chlorine held high for several days running. A clear pool rarely needs a second shock the next day.
- Can you shock a pool with liquid chlorine?
- Yes. Liquid chlorine is a chlorine shock, just a less concentrated one, so it takes more of it than granular shock does.
- Should the pump be running when you shock?
- Yes. The pump spreads the shock through the whole pool. Without it the chlorine sits in one spot and can bleach plaster or a liner.
Sources
Keep reading
How long after shocking a pool can you swim?
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How to get algae out of a pool
To clear a green pool: balance the pH to about 7.2, shock with roughly three times the normal dose after ...
Why does my pool smell?
A strong chlorine smell usually means chloramines, the irritating compounds chlorine forms with sweat and...
How much liquid chlorine should you add to a pool?
To raise free chlorine by 1 ppm in 10,000 gallons, add about 10 fluid ounces of 12.5% liquid chlorine, ab...
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