Why Is My Pool Cloudy? Causes and How to Clear It

A pool turns cloudy when tiny particles are floating in the water faster than the filter can take them out. Those particles come from only a handful of places: something living (algae or bacteria, because the chlorine is too low), something chemical (calcium coming out of solution, usually because pH is high), or something the filter is missing (dirt, pollen, dead algae, or a filter that isn’t working). Every one of these can be tested for, so you don’t have to guess.

The short version: test the water, fix free chlorine first, bring pH down into range, then run the filter nonstop until it’s clear. Most pools clear within one to three days once the real cause is fixed. The rest of this guide shows you how to tell which cause you have and what to do about each one.

Start with a full water test

Before adding anything, test these six numbers. Strips will do in a pinch, but a drop-based kit that measures free chlorine with the FAS-DPD method gives readings you can act on.

• Free chlorine (FC) and combined chlorine (CC)
• pH
• Total alkalinity (TA)
• Cyanuric acid (CYA), also called stabilizer
• Calcium hardness (CH)

Then check two things that aren’t on the test kit: the filter pressure gauge, and how many hours a day the pump actually runs.

Pool health checkerEnter your test results to see which numbers are out of range and what to fix first.

Why is my pool cloudy? The common causes

Low free chlorine and early algae

This is the most common cause, and the one to rule out first. Chlorine kills bacteria and algae before they multiply. When the level drops too low, they start growing, and the first sign is a dull, hazy look well before the water turns green.

“Too low” depends on your stabilizer. CYA protects chlorine from the sun, but it also holds most of it in reserve, so a pool with 50 ppm CYA needs far more free chlorine than one with 20 ppm. A reading of 2 ppm can be fine in one pool and an open door for algae in another. The CYA guide explains the relationship, and the chlorine calculator shows the range for your CYA.

If the water is hazy and chlorine is low, or chlorine keeps disappearing faster than you add it, treat it as algae: shock the pool to about 40% of your CYA level and hold it there. You’re done when three things are true: the water is clear, combined chlorine is 0.5 ppm or less, and free chlorine drops by no more than 1 ppm overnight.

Pool shock calculatorGet your shock level and how much of your product to add, from your CYA and pool size.

High pH and calcium (milky white water)

If chlorine is fine but the water looks milky, pH is the next suspect. When pH, calcium hardness and alkalinity are all on the high side, especially in warm water, calcium carbonate comes out of solution. It shows up first as a fine white cloudiness and later as rough scale on tile, steps and inside the heater. Test kit makers measure this tendency with the saturation index; Taylor considers water balanced between about −0.3 and +0.5, and above that range you get cloudiness first and scale later.

High pH also weakens chlorine, so the two problems often show up together. The CDC range for pH is 7.2 to 7.8.

To fix it:

1. Lower pH to about 7.2–7.4 with muriatic acid or dry acid, added in front of a return with the pump running. The pH calculator works out the dose.
2. If alkalinity is high (above about 120 ppm), it will keep pushing pH back up. Lowering it takes several rounds of acid and aeration; the pH guide walks through it.
3. If calcium is very high and the haze keeps coming back, partly draining and refilling with softer water is often the only lasting fix.

Salt pools see this more than most. A salt chlorine generator pushes pH up steadily, and scale builds on the cell plates.

Combined chlorine

Combined chlorine is chlorine that has already reacted with sweat, urine, sunscreen and other waste. A little is normal. When it climbs above about 0.5 ppm, the water can look dull and smell strongly of “chlorine.” The fix is more chlorine, not less: raise free chlorine to shock level and hold it until combined chlorine drops back down.

The filter and circulation

If the chemistry is in range and the water still won’t clear, the filter is the problem. Chlorine kills what’s in the water, but only the filter removes it.

• Run the pump around the clock until the water is clear. Most cloudy pools that “won’t clear” simply aren’t filtering long enough. Once it’s clear, go back to a normal schedule; the pump run time calculator works out how long that is for your pool.
• Check the pressure gauge. Write down the reading when the filter is clean. When it climbs about 8–10 psi above that, or whatever your filter’s manual says, it’s time to clean or backwash.
• Sand filters: backwash until the sight glass runs clear. Sand that’s old, clumped or “channeling” lets fine particles straight through and may need replacing.
• Cartridge filters: hose each pleat clean. If the pressure stays high right after cleaning, or the fabric is torn or flattened, replace the cartridge.
• DE filters: backwash and recharge with the right amount of DE. A puff of DE on the pool floor after recharging usually means a torn grid.
• Circulation: point return jets down and around the pool so there are no still corners, and brush the walls and floor daily while you’re clearing it.

Many above-ground pools come with small filters and short pump timers, which is why they cloud up easily in summer.

Fine debris: rain, pollen and heavy use

Heavy rain washes in dirt, pollen and organic matter, dilutes your chlorine and can shift pH. A busy pool party adds sunscreen, oils and sweat. Wildfire smoke, dust and nearby construction add particles too fine for most filters to catch on the first pass.

After a storm or a crowded weekend, test and correct chlorine and pH, then run the filter continuously. Most debris haze clears on its own within a day or two. If a light haze lingers after that, it’s the case where a clarifier can help.

Metals

If the haze has a color, especially green, teal or brown, and it appeared within minutes of adding chlorine, metals are a likely cause. Well water often carries iron or manganese, and copper can come from old algaecides or corroded heater parts. Chlorine oxidizes the metal and it comes out of solution. Shocking won’t fix it. Test for metals (most pool stores can), and use a metal sequestrant or remover as its label directs.

Cloudy after shocking?

Some cloudiness right after a shock is normal. The shock kills algae and breaks down waste, but the dead material stays in the water until the filter catches it. If you used cal-hypo, it also adds calcium and briefly raises pH, which can turn hard water milky for a while.

Keep the filter running nonstop, clean it when the pressure climbs, and brush and vacuum. The water should look better within a day and be clear within a few days. If it’s getting worse instead of better, check three things:

• Free chlorine dropped below shock level overnight, so the algae came back. Retest and top up.
• pH or calcium is high, especially if you used cal-hypo in hard water. Test and lower pH.
• The filter is overloaded. A big algae kill can clog a filter quickly. Clean it, maybe more than once.

Clarifier or flocculant: when to use them

Neither product fixes a chemistry or filter problem, so use them only after chlorine and pH are in range and the filter is working.

Vacuuming to waste needs a sand or DE filter with a “waste” setting, or a separate drain line. Most cartridge filters don’t have one, so floc is usually a poor choice there. Follow the label dose exactly; more clarifier is not better, and don’t use clarifier and floc together.

How to clear a cloudy pool, step by step

1. Test everything: free and combined chlorine, pH, alkalinity, CYA and calcium. Note the filter pressure.
2. Fix free chlorine first. Bring it into range for your CYA. If you see signs of algae, or chlorine disappears overnight, shock and hold.
3. Bring pH to 7.2–7.6, and start lowering alkalinity if it’s high.
4. Clean or backwash the filter, then run the pump 24 hours a day.
5. Brush the walls and floor daily, and vacuum anything that settles.
6. Retest daily and correct chlorine and pH as needed.
7. After two days of nonstop filtering, if a light haze is all that’s left, consider a clarifier. If you still can’t see the floor, consider floc.
8. Still cloudy after all of that? Have the water tested for metals and calcium, and have the filter checked. A torn cartridge, a broken DE grid or worn-out sand is a common hidden cause.

How to keep it from coming back

• Keep free chlorine matched to your CYA every day, not just when you remember. Sun, heat and swimmers use it up fast in summer.
• Keep pH between 7.2 and 7.8, and don’t let alkalinity run high.
• Run the pump long enough to turn the water over about once a day, longer in heat waves and after heavy use.
• Clean the filter when the pressure rises, not on a fixed date.
• Brush once a week, especially corners, steps and behind ladders where water barely moves.
• Test after storms and parties, when cloudiness usually starts.

Most cloudy pools come back to one of these, and most of them are fixed with chlorine, pH and patience with the filter.