Green water is not a mystery — it’s algae. Algae is everywhere, in the air and on every surface, and it will grow in any pool that offers it light, warmth, and nutrients while the sanitizer isn’t keeping up. A pool that “keeps” turning green is telling you one of those conditions is recurring. Here’s what to look at, in order.
The three conditions algae needs
- Light and warmth — in this climate, algae has year-round growing conditions. You can’t remove this one; you manage the other two.
- Nutrients — organic debris (leaves, seeds, dust), body oils, and anything that’s decomposing underwater. The more that sits, the more food there is.
- A sanitizer gap — free chlorine below working levels, chlorine locked up by high pH, or chlorine neutralized by cyanuric acid that’s too high.
The usual suspects
Sanitizer demand outpacing supply
Heavy use, a heat wave, or a debris load all raise the pool’s chlorine demand. If the chlorine feed can’t keep up — or there’s no one topping it off — there’s a window where the water is effectively unsanitized. That window is where algae starts.
Circulation dead spots
Algae loves the places the return jets don’t reach: the shallow end, corners, and the step. Water that lingers there gives algae time to attach. Brushing and proper return jet aim are the fix — not more chlorine alone.
pH drift
As pH climbs past about 7.8, chlorine becomes dramatically less effective. A pool can test “in range” on chlorine and still be under-sanitized because the pH is letting the chlorine sleep. See how often pool water should be tested for the cadence that catches this early.
Debris left to break down
A wind event can load a pool overnight. Debris that sits for days leaches organic matter into the water column and becomes a standing nutrient source. This is why wind-exposed pools — particularly along the coast and in open lots — turn green after events if the debris isn’t pulled quickly. See how Santa Ana winds affect pools.
Gaps in service
The most common cause of a recurring green pool is simple: the water goes unattended for long enough between visits that chemistry drifts and debris accumulates. If your pool turns green on a predictable cycle, the interval is too long for that property.
How a green pool gets treated
A proper restoration is a sequence, not a single dose of shock: the water is assessed, the pool is brushed, the filter is cleaned or serviced so it can handle the die-off, sanitizer is raised to knock the algae out, and the water is vacuumed as it clears. Cutting corners — especially skipping the filter work — is why pools bounce back green a week later. If yours is green now, that’s what a green pool cleanup assessment is for.
Keeping it from coming back
- Match the service interval to the property’s real debris and usage load.
- Keep pH in the working band so sanitizer actually works.
- Pull debris early, not on a “we’ll see how it looks” schedule.
- Keep the filter clean enough to hold the die-off load — signs your filter needs cleaning.
- If you’re on a salt system, confirm the cell is actually producing — a tired cell silently cuts your sanitizer supply. See saltwater pool maintenance basics.