Cloudy Pool Water: Working Through the Five Causes

Green water is the easier problem, because you know immediately that it is algae. Cloudy water is harder. It can be milky, grey, hard to see the floor through, and not green at all — and the possible causes lead in completely different directions.

This works through them from most common to least.

Cause 1: the filter is not doing enough

This is the most common cause and the most overlooked, because our instinct is to blame the chemistry first.

Some questions. How many hours a day does the pump run? A home pool generally needs at least 6-8, ideally during the day when the pool is being used. When did you last clean the pump strainer basket? When did you last backwash?

The part people forget is the filter media itself. Sand has a service life. After several years the grains wear smooth and channel, so water runs straight through without really being filtered.

If it is due for replacement, consider glass filter media instead of sand. It filters down to 4 microns, needs backwashing less often, cuts water use by around 25% and chemical use by up to half.

Cause 2: pH or hardness too high

Once pH climbs above about 7.8, dissolved calcium starts precipitating out as fine particles, and the water takes on a milky haze like suspended dust.

Test pH first. If it is high, bring it back to 7.2-7.6 with pH Minus and see whether the haze clears. In many cases that alone is the fix.

If pH is normal and it is still cloudy, suspect high calcium hardness, which is common where the mains water is hard.

Cause 3: sediment too fine for the filter to catch

Sometimes the particles in the water are simply smaller than the filter can trap, so they circulate indefinitely and no amount of running the pump clears them.

The fix is to make the small particles clump together until they are big enough to be filtered out or to sink.

To make them sink and vacuum them away, use Pool Floc at 400 g per 10,000 litres, or Fast Floc which works in 3-6 hours.

To have the filter catch them instead, use Pool Shimmer at 85 ml per 10,000 litres, or Maxi Clear skimmer tablets, which are very convenient on a sand filter — one sachet treats 50,000 litres.

One warning: if you use a flocculant, vacuum to waste, not through the filter. Set the valve to waste, otherwise the sediment goes straight back into the pool.

Cause 4: metals in the water

If the cloudiness leans towards faint green, brown or rust, and adding chlorine makes the colour stronger rather than weaker, suspect dissolved metals.

Metals usually come from bore water, old steel pipework or a heater. Test with a copper and iron test kit.

If metals are present, use Metal Magnet to bind them so the filter can remove them. If staining has already appeared on the surfaces, use Stain Remover at 1 kg per 50,000 litres.

Cause 5: heavy use, or heavy rain

This one is nobody's fault. Sunscreen, sweat, dust and rainwater all add load to the filtration system.

If the water clouded after a party or a storm, it will usually clear itself within a day or two. Run the pump longer and shock the pool once.

The order to work in

Check the filter, then test pH, then reach for a flocculant or clarifier, and if the colour looks odd test for metals.

Most importantly, do not throw several chemicals in at once hoping something works. If it clears you will not know what fixed it, and if it does not you have made the diagnosis harder.

Not sure which of these fits your pool? Message us on LINE, describe what you are seeing, and our technical team will help you work it out.