Why is my pool cloudy?

A cloudy pool is almost always one of four things: not enough free chlorine for your CYA, pH too high, algae getting started, or the filter not keeping up. Test free chlorine (FC), pH and CYA before adding anything. Those three readings sort most cloudy pools, and the order you fix things in matters more than the products.

What your test says

Test result	Likely cause	First move
FC below the minimum for your CYA	Not enough chlorine; algae can start	Raise FC to the target tonight (after any acid step)
Combined chlorine above 0.5 ppm, or more than 1 ppm FC lost overnight	Something is using up chlorine, often early algae	Shock and hold: see the green pool guide
pH 8.0 or higher	High pH clouds water and forms scale	Lower pH with acid, wait 30 minutes, retest
Chemistry all in range	The filter isn't keeping up	Clean or backwash the filter; run the pump longer

The minimum depends on CYA: 3 ppm at CYA 40, 5 ppm at CYA 60. The chlorine guide has the full table.

How to clear a cloudy pool, step by step

1. Test free chlorine, pH and CYA. Add combined chlorine (CC) if your kit reads it.
2. Keep swimmers out if FC is very low. Below half the minimum for your CYA (and always below 1 ppm), the water isn't sanitized.
3. Lower pH first if it is 8.0 or higher. Add acid separately, with the pump running, wait 30 minutes and retest. A reading of 8.2 means 8.2 or higher.
4. Raise free chlorine to the target at dusk. At CYA 40, aim for 6 ppm in a pool dosed by hand; a salt pool at CYA 70 also aims for 6. Add it separately, with the pump running.
5. Shock and hold instead if you see algae or CC is above 0.5. Hold FC at shock level (16 ppm at CYA 40, 28 at CYA 70) until the water clears and the tests pass.
6. Check the filter. If the pressure is high, clean or backwash it, and run the pump for at least one full turnover a day.
7. Retest FC at dusk and again at dawn. Losing more than 1 ppm overnight means something is still using up chlorine.

Chlorine per 1 ppm

Product	Per 10,000 US gal	Per 10,000 litres
10% liquid chlorine	12.8 fl oz	100 mL
12.5% liquid chlorine	10.2 fl oz	80 mL

Multiply by the ppm you need. For example, raising a 10,000-gallon pool from 3 to 6 ppm takes about 38.4 fl oz (5 cups) of 10% liquid chlorine.

Chemistry is fine, water is still cloudy

When FC, pH, alkalinity and CYA all read in range and the water is still hazy, the cause is usually mechanical: a dirty or worn filter, or too few pump hours. Clean or backwash the filter, run the pump longer, and keep chlorine at its target while the filter catches up. The pump guide shows how many hours one turnover takes.

Green, or cloudy after green

• Green water is algae. Shock and hold, as in the green pool guide.
• Cloudy after clearing algae is usually dead algae the filter hasn't caught yet. Keep filtering and keep chlorine at the shock level until all three finishing tests pass.

Metric and Australian notes

• In Australia, liquid chlorine is usually 12.5%: 80 mL per 10,000 litres raises FC by 1 ppm.
• Cartridge filters have no backwash; hose the cartridge clean instead.
• Salt pools use lower chlorine targets at the same CYA; the minimum is about 4.5% of CYA rather than 7.5%.

Questions people ask

Can I swim in a cloudy pool?

Not if free chlorine is below the minimum for your CYA, and not if you can't see the bottom of the deep end. Cloudy water hides someone in trouble.

Why is my pool cloudy after shocking?

Usually dead algae and debris the filter hasn't caught yet. Keep the pump running and the filter clean, and wait until FC is at or below shock level before swimming.

Why does my chlorine disappear overnight?

Losing more than 1 ppm overnight means something is using it up, often algae too small to see. That is a reason to shock and hold even when the water looks nearly clear.

My test strip says the water is fine. Why is it cloudy?

Strips are rough. Test FC, CC, pH and CYA with a drop-count kit before you treat; it's the most reliable way to read them.

For a plan built from your own readings, enter them in the water check.