WATER QUALITY
Identify a stain before you scrub it
Different stains need opposite treatments and the wrong one sets the mark permanently, so test it with a vitamin C tablet and a chlorine tablet to tell metal from organic from scale before you start.
Metal stains lift with acid; organic stains lift with chlorine. Use one on the other and the mark sets harder, sometimes for good. The two tests below take under five minutes and identify almost every stain you will meet.
| What it looks like | Usual cause | How to test it |
|---|---|---|
| Brown or rust-red | Iron in bore water, or a rusting fixing or fitting | Hold a vitamin C tablet against it for 30 seconds — if it fades, it is metal |
| Green, grey or black spots | Embedded algae, or a leaf left lying for days | Hold a chlorine tablet against it for 30 seconds — if it fades, it is organic |
| Blue-green over a broad area | Copper — usually a copper-based algaecide, or heat-pump internals | Vitamin C also fades it, but the mark is a smooth wash rather than spots |
| Grey-white and rough to the touch | Calcium scale, from pH and hardness left high for a long time | Put a drop of dilute acid on it — if it fizzes, it is scale |
Metal stains
Bring the pH down to 7.2 before you start.
Use a pool-grade stain remover at the rate on its label.
Add a metal sequestrant as soon as the mark fades, so the iron you released does not re-deposit somewhere else.
Stop topping the pool up with bore water — that is nearly always where the iron came from.
Organic stains
Brush the area well with a soft nylon brush.
Shock the pool in the evening, then run the pump for 24 hours. Most of these are gone by morning.
Calcium scale
Bring the pH down to 7.2 and hold it there.
Test calcium hardness and get it under 400 ppm.
Wait. A light film dissolves back over two to three weeks; a hard band at the waterline needs a technician with the right abrasive.
Whichever type it was, hold the water steady for at least a fortnight once the mark has faded. Stains nearly always come back because the chemistry swings, not because you did not scrub hard enough.
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