In most parts of the country, winter means closing the pool. In Naples, Marco Island, Bonita Springs, and Estero, winter means 68-degree days, 48-degree nights, and a sneaky chemistry shift that catches even experienced pool owners off guard.
You’re still swimming on sunny afternoons, but overnight the water temperature can drop 10–15 °F. That swing is enough to slow everything down — chlorine demand, circulation, and your sanitizer’s killing power — while algae still grows happily (because our daytime highs are still in the mid-to-upper 70s).
By February, we get the same call every year: “My pool was perfect in October. What happened?”
Here’s exactly what happens when those cooler nights arrive — and the five preventable mistakes that turn crystal-clear water swamp-green before spring.
1. Chlorine Works Slower in Cooler Water
Fact: For every 10 °F drop in water temperature, chlorine’s effectiveness drops roughly in half.
When your pool dips into the low 70s or upper 60s at night, the residual chlorine you had in September is no longer strong enough to fight the organic load (sunscreen, pollen, leaves, oak tassels, and dust that blow in all winter long).2. Reduced Pump Run Time Kills Sanitizer LevelsMany owners cut pump run time in winter to “save electricity.” That saves pennies but costs hundreds in chemicals and labor later. Shorter run times mean:
- Less filtration of debris
- Poor chemical distribution
- Stagnant corners where algae quietly set up camp
3. Phosphates Love Our Winter Weather
Phosphates (algae food) wash into the pool every time it rains — and we get plenty of pop-up showers December through February. Combine high phosphates with slower chlorine and you have a perfect algae incubator.
4. pH Creeps Up When You’re Not Looking
Cooler water holds more dissolved carbon dioxide, which actually helps keep pH stable… until you shock the pool or add trichlor tabs (both very acidic). Then pH crashes, calcium flakes out, and the water clouds up — setting the stage for a green takeover.
5. You Stop Testing Because “It’s Winter”
This is the #1 reason pools go green in February. Out of sight, out of mind — until the water looks like pea soup.
How to Keep Your Pool Sparkling All Winter Long (Naples-Style)
- Maintain 2–4 ppm free chlorine year-round
Switch to liquid chlorine or raise your salt cell output when water drops below 78 °F. - Keep total alkalinity 80–100 ppm
This buffers pH swings when temperature fluctuates. - Run your pump a minimum of 8–10 hours daily (even in winter)
Variable-speed pumps at low RPM are inexpensive to run and keep water moving. - Shock weekly with liquid chlorine (not cal-hypo or dichlor)
Liquid doesn’t add stabilizer or calcium — perfect for Florida’s already high CYA and calcium levels. - Test and balance twice a week
Takes 3 minutes and prevents 3 weeks of headache. - Add a phosphate remover in November and again in January
Keep phosphates below 300 ppb and algae starves, even if chlorine dips temporarily. - Brush and skim religiously
Oak tassels and dust are everywhere from December to March. Stay ahead of it.
The Bottom Line
Cooler nights don’t mean less work — they mean different work. Treat your pool like it’s still summer (just with a light sweater) and you’ll swim in clear water all winter long or let Summit Pools handle it for you. Our winter weekly service keeps chemistry dialed in, equipment protected, and your weekends free — even when that random cold front drops the temperature into the 40s.
Ready for a worry-free Naples winter?
Call Summit Pools at (239) 228-6125 or schedule online at summitpoolsnaples.com.
We keep Southwest Florida swimming 365 days a year.