What the colour of your pool is telling you

This is the common one. Algae has taken hold because there was not enough sanitiser to stop it.

It usually follows a pattern: a hot week, a pump running fewer hours than it needs, or a holiday. Algae doubles fast once the water warms up.

The fix is not just chlorine. The water has to be balanced first so the chlorine can work, then shocked properly, then filtered hard and vacuumed to get the dead algae out. That last step is the one people skip, and it is why a pool goes cloudy grey instead of clear.

If it has gone properly green, that is green pool recovery.

This is the one that wastes people's money, because it looks identical and is not algae at all.

Dissolved metals — usually copper, sometimes iron — oxidise in the water and tint it green. Chlorine does not remove them. Adding more chlorine can actually make it worse, because oxidising the metal is what brings the colour out.

Copper gets in from bore or tank water, from some algaecides, and from a corroding heater element where the water has been left aggressive for too long.

The tell is the chlorine reading. Algae-green water almost always tests low or zero. Metal-green water tests normal.

Cloudy water is a filtration or balance problem far more often than a sanitiser one.

The usual suspects: the pump is not running long enough to turn the water over, the filter is dirty or past it, or calcium and pH have drifted far enough that the water is dropping fine particles out of suspension.

Milky white specifically often points at calcium. Very fine particles that the filter cannot catch.

Before you dose anything, check how many hours the pump is actually running and when the filter was last serviced. A pool that cannot circulate cannot clear.

Brown water is nearly always iron rather than dirt.

It shows up after a top-up from bore water, or after heavy rain has washed run-off in, or where old metal plumbing is breaking down.

Tannins are the other cause and they look similar — that is leaf litter, common on a pool under heavy trees that has been left with a load of leaves sitting in it.

Both need treating differently from algae, and both usually need the water partly diluted rather than dosed.

Dark spots that will not brush off are usually one of two things: black algae, or metal staining.

Black algae is stubborn. It puts roots into porous surfaces like render and pebble, and a normal shock will not shift it. It needs brushing to break the surface layer before anything else will get at it.

Metal staining sits in the surface rather than on it, and brushing does nothing at all.

Getting this one wrong is expensive. People shock a stained pool repeatedly, which does not work, and eventually resurface something that only needed treating.

Water can look perfect and still be wrong.

Clear but stinging usually means chloramines or drifted pH, neither of which changes how the water looks. We have written that up separately in red eyes and the chlorine smell.

Clear water can also be aggressive — short of calcium, quietly taking minerals out of your surfaces. You cannot see that at all. It only shows up years later in the grout and the tile line.

Clear is not the same as balanced. It is why we test rather than look.

First, brush a patch of the wall and watch. If a green cloud lifts off and drifts, that is algae. If nothing moves and the water stays evenly tinted, suspect metals.

Second, test the chlorine before you add anything. Algae drives chlorine to zero. A metal problem usually leaves a normal reading.

For a stain, the commonly used trick is to hold a vitamin C tablet against it for half a minute. If the stain lightens, it is metal. If it does not, it is organic and chlorine is the right tool.

None of these are laboratory tests. They are enough to stop you treating the wrong problem for a fortnight.