Cloudy pool water — Haarlem, North Holland

Cloudy or milky pool water is one of the most common pool problems and usually signals a chemical imbalance, a filtration issue, or both. The cloudiness is caused by microscopic particles suspended in the water — too small to be caught by the filter or present in such quantity that the filter can't keep up. The first thing to check is free chlorine level: if it's below 1 ppm, bacteria and algae begin multiplying, producing the haze (this is why pools often go cloudy after a party with heavy bather load, a rainstorm that dilutes sanitizer, or when the chlorinator runs dry). Next is pH: when pH climbs above 7.8, calcium and other minerals start precipitating out of solution as microscopic crystals, creating a chalky haze that no amount of chlorine will fix. High calcium hardness (above 400 ppm) compounds this effect. On the filtration side, a worn-out sand charge (sand filters should be recharged every 5–7 years), a cracked DE grid, or a cartridge filter past its useful life can all allow particles to pass through. A filter running too few hours per day is equally culpable — the entire pool volume should turn over at least once every 8–12 hours. Poor circulation from a partially blocked return line, an undersized pump, or closed valves creates dead zones where particles settle and cloud the water. Finally, early-stage algae (before the water turns visibly green) presents as a dull, hazy appearance, particularly noticeable when looking at the pool floor from the surface. A pool technician will test a comprehensive water sample (free chlorine, combined chlorine, pH, alkalinity, calcium hardness, CYA, phosphates), inspect the filter and pump system, and develop a treatment plan that may include shock chlorination, clarifier or flocculant treatment, filter cleaning or media replacement, and a circulation review.