What Causes Cloudy Pool Water, and When It's the Surface Itself

Cloudy pool water is usually caused by one of four things: unmet chlorine demand, pH or alkalinity drift, insufficient filtration, or an algae bloom getting started. Fix those and most pools clear in days. But if the cloudiness keeps coming back with the chemistry verifiably correct, look at the fifth cause, a chalking gel coat that continuously sheds oxidized particles into the water. The white-towel wipe test tells you in ten seconds which problem you have.

The Usual Suspects: Four Causes You Rule Out First

Before anyone blames the surface, including us, the ordinary causes have to be eliminated, because they account for the vast majority of cloudy pools:

• Chlorine demand. When sanitizer can't keep up, after heavy swimming, a rainstorm, or a hot stretch, fine organic particles build faster than they're oxidized, and the water goes dull, then milky. Test free chlorine; if it's low or zero, that's your answer today.
• pH and alkalinity drift. High pH pushes dissolved minerals toward cloudiness and cripples chlorine's killing power at the same time. Get pH and total alkalinity back into range before judging anything else, our fiberglass water chemistry guide has the target numbers.
• Filtration shortfall. A filter can't clear water it never sees. Undersized runtime (most pools need roughly 8-12 hours a day in swim season), a cartridge past its life, sand that's channeled, or a pressure reading way off baseline all leave fine particles in suspension.
• Algae bloom onset. Water often turns hazy a day or two before it turns green, the bloom is starting, and dullness is the early symptom. If chlorine demand suddenly spikes alongside the haze, treat it as algae now, not after it colors up.

The Fifth Cause: When the Pool Itself Is the Contaminant

Gel coat that has oxidized past a certain point, from years of UV and sanitizer exposure, stops being a sealed surface and becomes a powder factory. The surface chalks: it sheds microscopic oxidized particles into the water continuously, the way old car paint chalks onto a rag. You shock, the water clears for a bit, and the haze creeps back, because the source was never in the water. It is the walls. Our guide to fiberglass pool chalking covers the mechanism in full.

A failing surface feeds the problem a second way: oxidized, worn gel coat is porous, and porous surfaces give algae anchor points that sanitizer struggles to reach. If you're fighting recurring blooms in the same spots, or can't tell whether a patch is algae or a stain, the surface may be hosting the fight you keep losing.

The Elimination Sequence: Four Steps, In Order

Run these in order and you'll have your diagnosis without guessing:

1. Test and correct the chemistry. Free chlorine, pH, total alkalinity, with fresh reagents or a pool-store lab test, not year-old strips. Correct anything out of range, run the pump, give it 48-72 hours. Most cloudy pools end here.
2. Audit the filter. Check the pressure gauge against clean baseline, inspect the cartridge or media, and count actual daily runtime. Clean or replace what's spent, extend runtime, give it a few days of full circulation.
3. Do the white-towel wipe test. Chemistry correct, filter sound, water still hazing? Take a clean white towel and firmly wipe a dry section of wall at the waterline, or an underwater wall section with your hand in the towel. Look at the towel.
4. Read the towel. Clean towel: keep hunting in the water, retest, and consider a fine particulate issue your filter needs help with. Chalky residue on the towel: the surface is the source. The gel coat is oxidized and shedding, and it will keep clouding every fill until the surface itself is restored.

What Fixing the Surface Actually Means

One tempting shortcut fails every time: draining and refilling. Fresh water looks great for a week or two, then the same surface sheds the same chalk into it, and you've spent a water bill learning what the towel already told you. Worse, draining a fiberglass pool without the right precautions risks the shell itself, so it's not a diagnostic to run casually.

A chalking surface can't be shocked, sealed, or scrubbed back to health, the oxidation goes through the finish, not just on it. And painting over it fails fast, because coatings don't bond to chalk. The cure is fiberglass pool resurfacing done the way the shell was originally finished: the oxidized layer fully sanded off, repairs made, and factory-grade marine gel coat spray-applied so it chemically bonds to sound material. That surface is sealed, non-porous, and typically good for 15-25 years, which means the cloudy-water cycle, the algae anchor points, and the chalk on the towel all end with the same treatment. Water problems get water treatments; surface problems get surface treatments. The whole trick is knowing which one you've got, and now you do.