A green pool is algae, growing because chlorine ran too low. Brush the walls and floor, then shock: raise free chlorine to the shock level for your stabilizer — 40% of your CYA, and at least 10 ppm — and keep it there until the water is clear and chlorine holds overnight. Run the pump 24 hours a day and clean the filter often.
Scoop out leaves and debris, so the chlorine works on the algae and not on them.
2
Test free chlorine, pH and stabilizer. Bring pH into 7.2–7.6 first: chlorine works far better there.
3
Work out the shock dose with the calculator on this page and add it in the evening. Liquid chlorine works fastest and adds no stabilizer or calcium.
4
Brush everything twice a day, run the pump around the clock, and clean the filter every time its pressure climbs.
5
Test morning and evening, and add chlorine back up to shock level each time it falls.
6
You are done when the water is clear, chlorine drops by 1 ppm or less overnight, and no new green appears.
The product label always wins — if it says something different, follow the label.
Why it happens
Algae spores blow in all the time. Chlorine kills them as fast as they arrive — until it runs low after a hot week, heavy swimming or rain.
High stabilizer makes the same chlorine reading much weaker, so a pool can turn green with chlorine that looks fine on a strip.
Questions
How long does it take to clear a green pool?
Usually two to seven days, depending on how green it is and how well the filter works. Keeping chlorine at shock level the whole time is what makes it fast.
My pool is green but chlorine is high. Why?
Check your stabilizer (CYA). The more stabilizer, the more chlorine it takes to kill algae: the shock level is 40% of your CYA, so at 80 ppm CYA you need 32 ppm.
Can I swim in a green pool?
No. Wait until the water is clear and chlorine is back in its normal range.
Why did my pool turn green overnight?
Chlorine ran out — usually after heat, a party or heavy rain. Once chlorine is gone, algae can bloom within a day.