Cloudy pool water is caused by poor filtration, unbalanced pH or alkalinity, low chlorine, high calcium hardness, environmental contaminants, or lingering post-shock particulates. Fix it by cleaning the filter, running the pump 24 hours, balancing chemistry, and adding a clarifier. Most cloudy pools clear within 48 hours.
Cause 1: Poor filtration
The most common cause of cloudy water is a filter that can't keep up. That means the filter is dirty, the pump isn't running long enough, or the pump is under-sized for the pool. Check filter pressure first — if it's more than 8 PSI above baseline, backwash a sand filter or hose out a cartridge filter. Then extend pump runtime to at least one full turnover per day (usually 8–12 hours).
Cause 2: pH and alkalinity out of range
When pH climbs above 7.8, minerals dissolved in the water precipitate out as microscopic particles — a phenomenon called "cloudy from calcium." Bring pH down to 7.2–7.4 with dry acid, let it circulate for 24 hours, then rebalance. Low alkalinity (below 80 ppm) also causes cloudiness by allowing pH to swing wildly.
Cause 3: Low chlorine
When free chlorine drops below 1 ppm, waterborne bacteria and micro-algae start multiplying invisibly. You'll see the water turn dull and slightly hazy before you see actual algae. Shock the pool with liquid chlorine at 10× your combined chlorine reading and run the pump 24 hours.
Cause 4: High calcium hardness
Above 400 ppm calcium hardness, the water becomes supersaturated and calcium precipitates as a milky suspension. Westchester's fill water is moderately hard, so calcium tends to climb over multiple seasons on the same water. The only real fix is a partial drain and refill — usually 25–33% of the pool volume.
Cause 5: Environmental contaminants
Pollen (heavy in Westchester in May and early June), fertilizer runoff from a nearby lawn treatment, dust from construction, or a sunscreen bloom from a big pool party will all turn water cloudy overnight. Skim heavily, run the pump 24 hours, add a clarifier, and expect it to clear within 48 hours as the filter catches particles.
Cause 6: Post-shock cloudiness
After a heavy shock, water sometimes turns cloudy for 12–48 hours as calcium hypochlorite (Cal-hypo) releases calcium into the water or as dead algae particles suspend before the filter catches them. This is normal and clears on its own with continuous filtration. If it doesn't clear in 48 hours, one of the other five causes is layered on top.
The 48-hour clarifying protocol
- Test all chemistry. Bring pH to 7.2 and alkalinity to 80–100 ppm.
- Backwash or clean the filter.
- Add a clarifier (a polymer flocculant) per label instructions.
- Run the pump 24 hours continuously — do not turn it off overnight.
- Brush the pool at hour 12 to release any settled particles into the filter.
- At hour 24, backwash again — the filter will be loaded with the particles it caught.
- At hour 48, water should be visibly clearer. Retest chemistry and rebalance.
For truly stubborn cloudiness — usually a heavy calcium event — use a flocculant instead of a clarifier. Flocculant clumps the particles and drops them to the floor as sediment, which you then vacuum to waste (bypassing the filter). It works in 24 hours instead of 48 but requires a filter multiport valve and some skill.
When cloudy water is a warning sign
Cloudy water that returns within days of clearing usually points to an underlying equipment problem — a failing pump, a filter with channeling in the sand bed, or a plumbing air leak. If you've run the clarifying protocol twice in one month, stop guessing and get a technician on-site. It's almost always cheaper to diagnose the root cause than to keep buying chemicals.
Prevention
Weekly service, filter cleaned or backwashed on schedule, chemistry held tight, and the pump running long enough to turn the water over daily. Do those four things and you'll see cloudy water once or twice a season — right after big storms — and never for more than 24 hours.
Related service
See our pool repair service