Why Does My Pool Turn Green After It Rains?

Rain turns pools green because it does four things at once: it dilutes your chlorine and stabilizer, lowers pH and alkalinity, washes organic material and nutrients into the water, and often knocks out the pump. Any one of those alone is manageable. All four together will produce a bloom within 24 to 48 hours in warm weather.

What a heavy rain actually changes

Dilution. Two inches of rain into an average pool is thousands of gallons of unchlorinated water. It drops free chlorine and it drops cyanuric acid, and lower CYA means the chlorine you have left burns off faster in the sun the next day.

pH and alkalinity fall. Rainwater is naturally acidic, typically around pH 5.0 to 5.5. A big storm can pull a well-balanced pool down noticeably, and low alkalinity makes pH unstable afterward.

Nutrient load. Runoff from decks, planters, lawns and roofs brings in dirt, leaves, pollen, fertilizer and phosphates. Phosphates are algae food. Levels above about 500 ppb are worth addressing.

Circulation stops. Storms cause power outages, and a pool that sits with a dead pump for a day is a pool with no sanitizer distribution.

What to do before a storm

• Bring free chlorine up to the high end of your target range (not shock level, just the upper end) the day before
• Confirm pH is 7.4 to 7.6 and alkalinity is 80 to 120 ppm so the drop has room to absorb
• Clear the deck and skimmer baskets so debris does not get washed in
• Do not drain the pool ahead of rain; the water weight helps hold it in place in saturated ground

What to do after

Test as soon as it is safe. Expect chlorine to be low and pH to be down. Skim and remove debris quickly, because organic material consumes chlorine fast while it sits. Brush the walls, clean the skimmer and pump baskets, backwash or clean the filter, and run the pump continuously for at least 24 hours.

Then correct in order: alkalinity, then pH, then chlorine, then retest CYA once things have settled. If CYA came down significantly, adjust your chlorine target accordingly.

Storm season in the Keys

This is the single most common reason we get a green pool call in Key West. Our rainy season runs roughly May through October, we get intense short downpours, and it overlaps with the highest UV and warmest water of the year. So the rain strips your chlorine and the sun immediately punishes whatever survives.

Tropical systems add wind-driven salt spray, leaf litter and, sometimes, days without power. After a named storm we treat every pool as if the chemistry has been reset, because functionally it has. Standing untreated water afterward also becomes mosquito habitat quickly, which in the Keys is a public health matter, not just a nuisance.

Test the day after every heavy rain rather than waiting for your regular schedule, because that 24-hour window is where a clear pool becomes a green one.