Green pool water: causes and step-by-step treatment
- Green water = algae 9 times out of 10, encouraged by heat, a drifting pH or a lack of sanitiser.
- The treatment: brush, test, correct the pH, shock treatment, long filtration, re-test.
- The shock dose depends on your volume, your product and your current level — the CliniPool app calculates it.
- Allow 24 to 72 hours to get back to clear water, depending on how far it's gone.
Green pool water is almost always algae: micro-organisms that take off the moment sanitisation weakens, the pH drifts or a heat spike joins the party. The treatment comes down to six steps: brush the pool, test the water, correct the pH, run a shock treatment dosed for your volume, filter non-stop, then re-test. Allow 24 to 72 hours to get back to crystal-clear water. The one thing this page won’t give you is the amount of product: it depends on your pool, and that’s exactly what the CliniPool app calculates for you.
Why your water turned green
Algae are always present — in the air and in top-up water. They’re just waiting for a weakness, and there are four classic ones. Not enough sanitiser: chlorine (or bromine) that drops too low, even for 24 hours, is enough in summer. A drifting pH: too high, and it makes chlorine largely ineffective — the chlorine is there, but it’s stopped working. Heat: above 28 °C everything speeds up, both chlorine consumption and algae growth. And stabiliser that’s too high: tablet after tablet, the chlorine ends up “locked” and does nothing, even with an apparently decent reading.
That’s why green water “even though the chlorine is fine” is no mystery: the total chlorine number can look correct while the active chlorine is sitting at zero.
The treatment, step by step
The logic is always the same: prepare the ground, hit hard once, then let the filtration finish the job.
1. Brush and clean. Walls, floor, waterline, skimmers, pump basket. Algae clinging to surfaces resist the treatment far better: once dislodged, they’re exposed.
2. Test your water. pH, chlorine and stabiliser as a minimum. With the app, you dip a strip, take a photo, and the results are read for you.
3. Correct the pH first. It’s the step everyone skips — and the reason shock treatments fail. Bringing the pH down towards 7.0–7.2 makes your shock genuinely effective.
4. Shock treatment. The dose depends on your volume, your product (shock chlorine, and its concentration) and your current level.
5. Filter non-stop. 24/7 during the treatment. Dead algae turn the water whitish before disappearing: it’s the filter that captures them. Clean it as soon as the pressure climbs.
6. Re-test and lock it in. Water clear? Full re-test, flocculant if a milky haze lingers, and fix the original cause so it doesn’t come straight back.
Can you swim during the treatment?
During and right after a shock, swimming is best avoided — not because of a few splashes, but because the sanitiser is deliberately at a very high level. The exact wait depends on the dose and your pool: the app tracks the treatment in real time and shows you when swimming is ideal again, instead of a vague “24 to 48 hours” rule.
Still green after the shock?
Three explanations dominate: the shock was underdosed (very common with the generic doses found online), the stabiliser is too high and is neutralising the chlorine, or the filtration hasn’t run long enough. Re-test, check the stabiliser, and run another properly dosed round. If the water is green and the stabiliser is very high, the stabiliser is your culprit — the stabiliser guide (below, under “Also worth reading”) explains why.
Stopping it coming back
Green water is almost always the story of a slow drift: a pH creeping up quietly, a tablet dissolving too fast, a heatwave nobody planned for. One photo test a week in season, filtration matched to the temperature, and the app warning you when a parameter starts to drift — that’s all it takes to never see green again.
Frequently asked questions
How long does it take for green water to clear?
Between 24 and 72 hours depending on the algae stage, how effective the shock is and the filtration. Light-green water: often 24-48 h. Opaque "pea soup": up to 72 h with flocculation.
Do you need to drain a green pool?
Almost never. Even seriously green water can be rescued with a properly run shock treatment. Draining is reserved for extreme cases (sludge, years of neglect) and carries its own risks for the pool structure.
Can you swim in green water?
Best avoided: algae make surfaces slippery, the water can harbour bacteria, and the shock treatment that follows means a swimming break anyway.
Why is my water green when the chlorine reading is fine?
The classic suspect is stabiliser that's too high: the chlorine is there but inactive. A pH that's too high can also wipe out much of its effectiveness. Test pH and stabiliser, not just chlorine.
Sources : OMS — Guidelines for safe recreational water environments