Why your pool turns green after rain — and how to fix it

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It's the most common text we get from June through September: "It rained all weekend and now my pool is green — what happened?" Here's what happened, and what to do about it.

What rain actually does to your pool

• It dilutes your chlorine. A hard Florida downpour can add inches of fresh water in an hour. Your sanitizer level drops just from dilution — before anything else happens.
• It feeds the algae. Rain washes dust, pollen, leaves, fertilizer, and bird-everything off your screen enclosure and roof into the water. That's a buffet of algae food (phosphates and nitrogen).
• It crashes your chemistry. Rainwater is slightly acidic and has zero calcium or alkalinity, so pH and total alkalinity drift — and unstable water lets chlorine burn off faster in the sun.
• The sun finishes the job. The day after a storm is usually blazing. Low chlorine + food supply + heat + sunshine = green, sometimes in under 48 hours.

The fix, step by step

1. Empty the baskets and skim the debris first. Organic matter in the water keeps eating your chlorine while it sits there.
2. Brush everything — walls, steps, corners. Algae anchored to surfaces shrugs off chemicals until it's broken loose.
3. Test, then balance pH first. Chlorine works dramatically better at pH 7.4–7.6. Shocking an unbalanced pool wastes half the shock.
4. Shock hard. A light green pool typically needs a double dose; a swamp needs more. Do it at dusk so the sun doesn't burn it off.
5. Run the filter around the clock until the water clears, and clean the cartridge or backwash when pressure rises.
6. Re-test in 24 hours and rebalance. Expect a cloudy phase while the filter catches dead algae — that's progress.

When to call in help

If you can't see the bottom of the shallow end, if the green comes back within a couple of days, or if you'd just rather be swimming than shocking — that's a green-to-clean job. It's one of our specialties, and honestly, catching it early costs less than fighting it for two weeks.

The real cure is prevention: on weekly service we rebalance after every storm cycle, so the algae never gets its opening.