How to Fix a Green Pool Fast (Without Draining It)
Updated September 2026
Fast answer: Green means algae, and algae has one cure: chlorine, in quantity, held for long enough. Drop pH to 7.2–7.4 first, then shock hard with liquid chlorine or cal-hypo. A green pool usually needs two to three times a routine shock dose. Run the filter around the clock, brush every day, and keep chlorine up until it stops disappearing overnight. Most pools go green to clear in three to seven days, and almost none of them need draining.
Why it turned green
Algae spores are in your pool right now. They blow in, ride in on swimsuits, come down with the rain. They don't matter as long as free chlorine stays above zero. The bloom starts the moment it doesn't, and the reasons are boring and familiar: a week you skipped, a heatwave that chewed through chlorine faster than your schedule assumed, a pump running six hours when it needed ten, or cyanuric acid up at 90 quietly handcuffing every ounce of chlorine you added.
The shade tells you how much work you're in for. Teal water you caught early, and it's a one or two day job. Grass green is established, so plan on three to five. If it's dark enough that you can't see the bottom of the shallow end, you're looking at about a week of real effort. Plaster, fiberglass, vinyl, all of them come back from any of those without draining. Only a genuine multi-season swamp with a debris layer on the floor makes a drain-and-clean the cheaper choice.
The kill sequence
pH comes first, not chlorine. Chlorine kills algae much faster in the 7.2–7.4 band, so bringing pH down before you shock makes everything you add afterward work harder. Then dose, and dose properly. Under-dosing is why most green pools stay green for two weeks instead of four days. A real kill means holding free chlorine at roughly 30–40% of your CYA reading for hours, not touching that number once and walking away. For a pool at CYA 40, that's 12–16 ppm, which is two or three routine shock doses. Add it at dusk so the sun doesn't burn off half of it before it's done working.
Brush everything, daily. Walls, steps, the shadowed strip behind the ladder, the corner the return jet never reaches. Algae builds a slick protective film over itself and brushing tears that open so chlorine can get to the cells underneath. Leave the pump running 24 hours a day for the duration, not on its usual timer.
Here's the test that tells you when you're done. Read free chlorine at dusk, read it again at dawn. If the pool lost more than about 1 ppm overnight, something in there is still alive and eating it. Re-shock and repeat. When the overnight loss drops under 1 ppm, the algae is dead and the rest is just filtration.
From dead algae to clear water
Dead algae turns the water a milky blue-gray, and people panic at this stage thinking they've made things worse. They haven't. Gray is the win. The corpses are just too fine for a filter to catch quickly, so the job now is patience and runtime. Keep the pump going, backwash or pull the cartridge whenever pressure climbs 8–10 psi over its clean baseline, and vacuum the dust that settles. Vacuum to waste if your valve allows it, so none of it recirculates.
A clarifier clumps the fine stuff into filterable sizes over two or three days. Floc is the impatient option: it drops everything to the floor overnight and then you vacuum it out to waste, slowly, and top the pool back up. Either works. When the water clears, retest the whole panel, because a heavy shock usually leaves pH low and may have burned off some stabilizer. Then deal with whatever caused it. Fix the runtime, fix the CYA, fix the schedule, or you'll be reading this page again in a month.
FAQ
How long does it take to clear a green pool?+
Teal, one to two days. Established green, three to five. Swamp, about a week. What actually moves those numbers is dose size (most people add a third of what's needed), pump runtime (it has to be 24/7), and whether you brush daily.
Can I swim in a green pool?+
No. Set aside the algae itself: green water hides the drain and hides swimmers, which is the real danger, and blooms travel with bacteria that cause ear, eye and skin infections. Wait until you can see the bottom clearly and free chlorine has settled back to 1–3 ppm.
Should I drain a green pool?+
Almost never. Chlorine is cheaper than a truckload of water and far safer than an empty shell. With a high water table, a drained pool can float out of the ground, and vinyl liners shrink and wrinkle once they dry. Draining is for multi-season swamps with debris on the floor, not for green water.