How to Fix Cloudy Pool Water (Find the Cause First)

Cloudy water is a symptom with four different diseases, and the internet's favorite advice ("just shock it") only cures one of them. Spend five minutes diagnosing and you will clear the pool in a day instead of fighting it for a week.

The four causes, in order of likelihood

1. Sanitizer problem (low or ineffective chlorine)

Low free chlorine lets bacteria and early-stage algae multiply, and billions of microorganisms scatter light before they ever tint the water green. Test free chlorine first, and test cyanuric acid with it: with high CYA, chlorine can read "fine" on a strip while being too suppressed to sanitize (the full relationship is in the CYA guide).

The fix: raise free chlorine to shock level, roughly 10 times your combined chlorine reading or about 40 percent of your CYA number, using liquid chlorine dosed with the chlorine calculator. Add at dusk, run the pump all night.

2. Filtration problem (dirty filter, short run time)

The filter is what physically removes cloudiness; chemicals only kill things. A sand filter past its pressure window, a cartridge overdue for cleaning, or a pump running 4 hours a day cannot keep any pool clear. Check the pressure gauge (clean the filter at 8 to 10 psi over its clean baseline) and audit your run time against the one-turnover rule in how long to run your pool pump.

The fix: clean or backwash the filter, then run the pump 24 hours straight until clear, then return to a normal schedule.

3. Chemistry imbalance (pH, alkalinity, calcium)

High pH plus high calcium pushes calcium carbonate out of solution, a fine white haze that is literally limestone dust suspended in your water. High alkalinity keeps pH pinned high, sustaining the cycle. This cloudiness looks milky-white rather than dull-green.

The fix: test pH, alkalinity, and calcium hardness. Bring alkalinity to 80 to 120 and pH to 7.4 to 7.6 with doses from the chemical calculator, in the order explained in the chemistry order of operations. The existing haze then filters out over a day or two.

4. Fine particles (dust, pollen, post-storm debris)

After storms, nearby construction, or heavy pollen, particles smaller than your filter's catch size stay suspended indefinitely. Chemistry tests all come back normal, chlorine holds, and the pool still looks like weak milk.

The fix: a clarifier (coagulates fines into filterable clumps, gentle, takes 2 to 4 days) or a flocculant (sinks everything to the floor overnight for manual vacuuming to waste, fast, more work). Vacuuming to waste drops your water level, and refilling dilutes salt and stabilizer, recheck with the salt calculator afterward if you have a salt pool.

The 5-minute diagnosis

1. Test free chlorine and CYA. FC low for your CYA → cause 1. Shock tonight.
2. FC fine? Check filter pressure and recent run hours → cause 2 if either is off.
3. Filter fine? Test pH, alkalinity, calcium → cause 3 if pH is 7.8+ or calcium is 400+.
4. All normal, and it started after a storm or pollen drop → cause 4. Clarifier or floc.
5. Still ambiguous: do 1 and 2 together anyway, they fix 80 percent of cloudy pools.

Keeping it from coming back

• Hold free chlorine at the right level for your CYA, not the strip's generic range.
• Run at least one full turnover daily, more during heat waves and heavy use.
• Retest after every party, storm, or heat wave, the three great cloudiness triggers.
• Clean the filter on pressure, not on the calendar.