Algae growth requires: water, light, nutrients, and the absence of adequate disinfectant. At night, there is no UV light — but algae also does not need light to multiply. Algae cells that survived the day (because chlorine was already low) continue dividing throughout the night in warm water.
The process typically starts during the day: a hot afternoon depletes chlorine from 1 ppm to near-zero. By evening, algae begins multiplying. By morning, the visible green bloom is established. It "turned green overnight" but the conditions developed over the previous 12–24 hours.
Trichlor tablet floaters and automatic chlorinators need refilling. If the last tablet dissolved Monday afternoon, there may be zero chlorine by Monday evening — no protection overnight. Always check the tablet supply when you see the pool.
Rain dilutes pool chemistry. A 2-inch rain event on a 15,000-gallon pool adds roughly 1,800 gallons of unchlorinated water — dropping free chlorine by 10–12%. Combined with the chemical dilution from the fresh water, it can push chlorine from 1.5 ppm to near zero. Add phosphates and nitrates from the rain, and algae has everything it needs.
At 95°F water temperature with direct sun, free chlorine can drop 3–5 ppm in a single day. A pool that started at 1.5 ppm on a hot July day may be at zero by 4pm. After-hours swimming in a warm, zero-chlorine pool accelerates the algae bloom.
When CYA is above 80 ppm, free chlorine is so tightly bound that it has virtually no sanitizing power. The pool reads "2 ppm free chlorine" on a test, but effectively has near-zero active sanitizer. Algae blooms despite the number looking fine.
The difference between a pool that can handle a hot July day and one that cannot is usually not the weather — it is the chlorine reserve and CYA level. Maintain 2–3 ppm free chlorine (not just 1 ppm), keep a floater with tablets for the overnight baseline, and keep CYA below 80 ppm.
PoolLens shows you when free chlorine is trending toward zero — before the overnight green happens. Set a low-chlorine alert and never wake up to a green pool again.
Open PoolLens Free →Yes — in warm weather with zero free chlorine, a pool can develop visible green color within 12–24 hours. The process typically starts during the day when chlorine is depleted, becoming visible by morning. It rarely happens purely overnight without a preceding daytime depletion event.
Rain dilutes pool chemicals — including free chlorine. A heavy rain event can drop free chlorine significantly while adding phosphates, nitrates, and algae spores from the atmosphere. This combination of diluted chlorine plus algae nutrients triggers rapid green-up.
Overnight green is triggered by: zero or near-zero free chlorine, warm water above 80°F, recent rain event, high phosphate levels, high CYA preventing chlorine from working, and heavy algae spore load from surrounding vegetation.
Maintain free chlorine above 2 ppm (not just 1), use a tablet floater for overnight reserve, test after every rain event, shock weekly in summer, keep CYA below 80 ppm, and verify the chlorinator has tablets weekly.
If your test shows chlorine but the pool is green, CYA is likely too high (chlorine lock), or the test kit or strips are old/inaccurate. High CYA creates a situation where chlorine reads as present but is bound so tightly it cannot kill algae. Test CYA and consider a partial drain.