Why is my pool cloudy? Causes and how to fix each one
Cloudy water means something is suspended in it that's too fine for the filter to catch or is forming faster than it's removed. It's usually one of five things. Test first, and the readings will tell you which.
Start by testing
Test free chlorine, combined chlorine, pH, total alkalinity, calcium hardness and stabilizer (CYA). Then work down this list, which runs roughly from most to least common.
1. Chlorine too low for your stabilizer
Signs: free chlorine below about 7.5% of your CYA (below 3 ppm at 40 ppm CYA), combined chlorine above 0.5 ppm, a dull or slightly green-white haze, slippery walls.
What's happening: early algae and bacteria are growing faster than the chlorine kills them. This is by far the most common cause, and often appears before the water turns visibly green.
Fix: shock to about 40% of your CYA and hold that level until the water clears, combined chlorine is 0.5 ppm or less, and the pool loses no more than about 1 ppm of chlorine overnight. Follow how to shock a pool and use the shock calculator.
2. High pH, alkalinity or calcium (scale in the water)
Signs: pH above about 7.8, high alkalinity or calcium, a white or milky haze, often after adding calcium, cal-hypo or soda ash, or in warm weather. Rough deposits on tile or in the salt cell.
What's happening: the water is oversaturated with calcium carbonate, and tiny particles form in the water itself.
Fix: lower pH to about 7.2–7.4 with acid, then keep it in range. Check the LSI: if it's well above +0.3, lower alkalinity or, for very hard water, replace some water. The haze usually filters out over a day or two once the water is no longer oversaturated.
3. Filter problems
Signs: chemistry looks fine, but the water stays hazy. High or very low filter pressure, short pump run times.
Fix:
- Run the pump longer, at least one full turnover a day, continuously while clearing. The turnover calculator shows how long that takes.
- Clean or backwash the filter when the pressure is about 8–10 psi above its clean reading, or as the manufacturer recommends.
- Check the filter media: torn cartridges, channeling sand or missing DE all let fine particles through. Sand typically needs replacing every several years.
- Make sure the multiport valve is on "Filter" and the skimmer and pump baskets are clear.
4. Fine debris after a storm, pollen or a pool party
Dust, pollen and swimmer load all add fine particles and use up chlorine. Bring chlorine up first, then filter continuously. A clarifier helps the filter catch fine particles. A flocculant clumps them so they sink and can be vacuumed out to waste. That works best with a sand filter or a vacuum that can bypass the filter. Follow the product label closely.
5. New water or a recent big chemical dose
Fresh fill water, especially from wells, can carry minerals and metals that cloud or color the water when chlorine is added. Large doses of baking soda, calcium chloride or cal-hypo can cause short-lived cloudiness. This usually clears with filtering within a day or two. If the water turns tinted green, brown or blue-green while clear, metals may be the cause, and a metal sequestrant may help.
A quick checklist
- Test free and combined chlorine, and compare with your CYA. If chlorine is low, shock and hold.
- Test pH. If it's above 7.8, lower it to about 7.4.
- Check the LSI. If it's high, lower pH, then alkalinity.
- Clean the filter, then run the pump continuously.
- Still cloudy after two or three days with good chlorine and pH? Use a clarifier, check the filter media, and test for metals.
Frequently asked questions
Is it safe to swim in a cloudy pool?
Not if you can't see the bottom clearly: it's a drowning risk, because a swimmer in trouble below the surface may not be seen. Cloudiness from low chlorine also means germs may not be controlled. Wait until it's clear and chlorine is in range.
Why is my pool cloudy after shocking?
Dead algae and particles make the water cloudy for a while, and cal-hypo can add a short-lived haze. Keep filtering and hold chlorine at the shock level until the overnight loss test passes.
Can high stabilizer make the pool cloudy?
Not directly, but high CYA makes chlorine less effective, so algae can grow even with a "normal" chlorine reading. That's cause number 1.