Green water means algae has established itself. It is never a mystery and almost never requires draining the pool. It does require doing four things in the right order.
What actually caused it
- Free chlorine hit zero — the most common cause, usually after heavy rain, a party, or a week of skipped service.
- pH drifted high — at pH 8.0 your chlorine is barely working even when the test says there is some present.
- Circulation was cut — running the pump three hours instead of eight leaves dead zones where algae settles.
- Chlorine lock — cyanuric acid overdosed, so the chlorine present is chemically unavailable.
The correct recovery sequence
Step 1 — Test everything. Free and combined chlorine, pH, alkalinity, cyanuric acid. Do not dose blind.
Step 2 — Correct alkalinity, then pH. Bring pH to 7.2 to 7.4 before shocking. Shocking at high pH wastes most of the chemical.
Step 3 — Brush every surface. Algae has a protective film. Chlorine cannot reach it through that film. Brushing is not optional and it is the step people skip.
Step 4 — Shock hard, at dusk. Sunlight destroys unstabilised chlorine within hours. Evening dosing gives it the whole night to work.
Step 5 — Run circulation continuously. Twenty-four hours minimum, with filter attention as pressure climbs.
Step 6 — Backwash, then repeat testing. Dead algae becomes filter load. Expect two or three backwash cycles.
Step 7 — Clarify and vacuum. Once the water is grey-blue rather than green, dead algae needs removing, not just filtering.
Why it keeps coming back
If the bloom returns within weeks, the underlying cause was never fixed — usually cyanuric acid too high, phosphates feeding the algae, or circulation hours still too short. A one-off rescue clean without correcting these buys you a month.
The cheaper path
Blooms are a monitoring failure. A pool with weekly documented chlorine and pH readings essentially does not turn green. Book a free pool health check to find out which of the four causes applies to yours.