How to Fix a Green Pool Fast (Without Draining It)
Updated September 2026
Quick answer: A green pool is an algae bloom, and the fix is always the same: balance pH to 7.2–7.4, shock heavily with liquid chlorine or cal-hypo (green usually needs 2–3× a normal shock dose), run the filter 24/7, brush daily, and keep chlorine elevated until the water turns cloudy-blue, then let filtration finish the job. Most pools go green-to-clear in 3–7 days without draining.
Why it turned green
Algae blooms when free chlorine hits zero long enough for spores — which enter every pool constantly via wind, rain and swimsuits — to take hold. The usual suspects: a skipped week, a heatwave that burned chlorine faster than the schedule, a filter running too few hours, or high cyanuric acid quietly handcuffing the chlorine you did add.
The shade of green is a severity gauge: teal means you caught it early (1–2 days of work), grass green is established (3–5 days), and black-green swamp means a week of aggressive treatment. All of them are fixable without draining in a plaster or fiberglass pool; only a true swamp with debris layers sometimes justifies a drain-and-clean.
The kill sequence
Step one is pH, not chlorine: lower pH to 7.2–7.4 first, because chlorine kills algae dramatically faster in that band. Then shock — and under-dosing is the number-one reason green pools stay green. A true algae kill needs free chlorine held at roughly 30–40% of your CYA level for hours, which for a typical pool means 2–3 normal shock doses of liquid chlorine or cal-hypo, added at dusk so sunlight doesn't burn it off.
Brush everything — walls, steps, behind ladders — because brushing breaks the algae's protective biofilm so chlorine can reach it. Run the pump 24 hours a day through the entire process. If the water is still consuming chlorine overnight, the algae isn't dead: re-shock and repeat until chlorine holds overnight.
From dead algae to clear water
Once algae dies the pool turns from green to a milky blue-gray — that's victory, not a new problem. Dead algae is too fine for lazy filtration, so keep the pump running continuously, backwash or clean the cartridge whenever pressure rises, and vacuum the settled dust (to waste, if your setup allows, so it doesn't recirculate).
A clarifier can speed the last stretch; flocculant is the nuclear option that sinks everything to the floor for a slow vacuum-to-waste. When the water clears, retest everything — a heavy shock usually leaves pH low and may have consumed stabilizer — and fix the root cause, or you'll do this again next month.
FAQ
How long does it take to clear a green pool?+
Teal-green: 1–2 days. Established green: 3–5 days. Swamp: about a week. The variables are shock dose (most people under-dose), pump runtime (needs to be 24/7), and daily brushing.
Can I swim in a green pool?+
No — besides the algae itself, green water hides drains and swimmers, and the bacteria that accompany blooms can cause ear, eye and skin infections. Wait until the water is clear and chlorine has returned to the 1–3 ppm range.
Should I drain a green pool?+
Almost never. Chlorine is cheaper and safer than draining — a drained pool risks popping out of the ground with high groundwater, and liner pools can shrink. Drain only for true multi-season swamps with heavy debris.