Why is my pool cloudy? Causes and fixes

Cloudy water is a symptom, and there are only a handful of usual causes. Work through them in order, most likely first.

Before anything else: if you can't see the bottom of the pool, nobody swims until it clears. CDC's pool code closes public pools when the main drain can't be seen from the deck, because a swimmer in trouble would be invisible. The same logic applies in your backyard.

1. The filter isn't keeping up

Taylor, Monterey County and HASA all list poor filtration or circulation as a cause, and it's the first thing to rule out. Check three things.

Look at the filter's pressure gauge. Pentair and HASA both say a filter needs cleaning when the pressure reads 8 to 10 psi above its clean starting pressure. Backwash a sand or DE filter, or pull and hose off a cartridge.

Check that water is actually moving. Empty the skimmer and pump baskets, and make sure the water level reaches about the middle of the skimmer opening.

Then look at run time. Pool water should pass through the filter about once every 6 to 8 hours (Taylor and South Australia's health department both use that figure). Sources split on how many hours to run each day. The Department of Energy suggests starting at 6 and adjusting, while HASA says 8 to 12. When the water is cloudy, skip the debate and run it around the clock until it clears.

2. Chlorine is too low

When chlorine runs low, the stuff swimmers bring in (sweat, sunscreen, oils) builds up faster than it's destroyed. Test free chlorine. If it's below 1 ppm, or below 2 ppm in a pool with stabilizer, raise it with the chlorine calculator.

For a 15,000-gallon pool, getting from 1 to 3 ppm takes about 1 qt 6 fl oz of 10% liquid chlorine.

If the water smells strongly of chlorine but tests low on free chlorine, test total chlorine too. A big gap means combined chlorine is high, and the shock calculator will tell you how much to add.

3. pH or calcium is high enough to scale

When pH climbs above about 8, calcium starts coming out of the water as tiny particles of scale. Shasta County and San Francisco both describe the result the same way: scaling, and cloudy water. High alkalinity and high calcium hardness push in the same direction.

Test pH, alkalinity and calcium. If pH is high, lower it with the pH calculator. If alkalinity is high too, fix it first, since it drives pH back up (more on that in the guide to lowering pH).

4. Early algae

A green tint, or haze that won't go away no matter how much you filter, can mean algae is starting. Shock now while it's easy. The green pool guide walks through it.

5. A busy weekend or a storm

Heavy swimmer use, rain and blown-in debris all load the water at once. Taylor and HASA both list this, and South Australia's health department says to shock after heavy use or rain. Run the filter continuously and clean it once it's caught up.

6. Dead algae after shocking

If you just shocked a green pool and it turned cloudy, that's often dead algae in the water. Keep the filter running and clean it as the pressure rises.

Clarifier or flocculant?

Both clump fine particles together. They work very differently after that.

Clarifier makes particles big enough for your filter to catch. It works slowly, and you just keep filtering. Taylor's advice is worth repeating: clarifier can help in the short term, but it doesn't fix whatever caused the cloudiness.

Flocculant makes particles heavy so they sink to the floor. The HTH and GLB labels follow the same routine: dose it, circulate about 2 hours, turn the pump off, let it settle overnight, then vacuum the floor to waste. Vacuuming floc through the filter just puts it back in the pool. That means floc only works if your filter has a waste or drain setting, which usually means a sand or DE filter with a multiport valve. Vacuuming to waste also drops the water level, so top up before you start.

Doses vary by product. Follow the label.

Keeping water clear

Most clear pools share the same habits: chlorine held in range, pH between 7.4 and 7.6, a filter cleaned before it clogs and enough run time to turn the water over. Write your readings on the weekly test log and you'll spot trouble a few days before the water does.