Most pool stains come from what has got into the water, not from a failing surface. Leaves, metals, scale and rust each leave a different mark, and each has a different fix. A stain only points to a surface problem when it will not shift after the water is balanced and the surface is treated, or when it sits in a rough, flaking or hollow patch.
Start by working out what kind of stain it is
The colour and the location tell you most of what you need to know. Here is how we sort them.
| What you see | Likely cause | Usually fixed by |
|---|---|---|
| Brown, tan or tea-coloured patches, often under trees | Organic: leaves, gum nuts, tannins | Brushing, sanitiser, enzyme or stain treatment |
| Green-blue or turquoise marks | Copper from pipes, heaters or algae treatments | Metal sequestrant, water balance, careful treatment |
| Brown, rusty or reddish marks | Iron from fill water, bores or fittings | Metal treatment and balanced water |
| White, grey or crusty deposits | Scale from high calcium or pH | Water balance, scale treatment, light acid wash |
| Small rust-coloured spots in the surface | Rebar or metal close to the surface | Usually needs the surface repaired |
| Black or dark spots | Algae or organic build-up | Brushing, sanitiser, then check circulation |
Organic stains: leaves and trees
In leafy streets, leaf litter that sits on the floor of the pool breaks down and leaves brown, blotchy stains, often worst on steps and in corners where leaves collect. Gum trees and similar species are common culprits. These stains are normally surface-level and respond to brushing, regular skimming and the right treatment. The best fix is prevention: keep leaves out, and empty skimmer baskets often.
Metal stains: copper and iron
Metals in the water can bond to the surface and leave coloured marks. Copper shows up as green or blue-green, iron as brown or rust. They commonly come from corroding heater elements, fittings, bore water or some algae treatments. A water test, then a metal-removal treatment and proper balancing, will usually deal with them. If the source is not found and fixed, the stains return.
Scale and calcium
White or grey crusty patches are calcium scale. It builds when calcium hardness or pH runs too high, and it is often heaviest at the waterline. Scale makes the surface feel rough and traps dirt. Light scale can be treated by correcting the water balance and, where appropriate, an acid wash. Heavy, long-standing scale is a different matter, and we cover how it ties into roughness in our guide to a rough pool surface.
Getting the diagnosis right before you treat
The most common mistake is treating a stain before knowing what it is. Throwing the wrong chemical at a metal stain can set it, and a generic shock treatment will not shift scale. A few simple steps help:
- Get a proper water test, including pH, alkalinity, calcium hardness and metals, not just chlorine
- Note where the stain is and when it appeared, for example after a storm or a refill
- Try a small, hidden test area before treating the whole pool
- Fix the source as well as the stain, whether that is a tree, a fitting or the water balance
Pool shops can test the water, and most stains respond to the right product used correctly. Take your time, because treating the wrong thing wastes money and can make a surface worse.
Rust spots from rebar
Small rusty spots in the surface itself, especially ones that return after cleaning, can mean steel reinforcing is too close to the surface and is rusting. Cleaning will not solve this. The area has to be cut out, the steel treated and the surface patched. If you see this, it is worth an inspection.
When a stain is a sign the surface has failed
Cleaning and chemistry are the first step for any stain. Think about the surface itself when:
- The stain returns quickly, in the same place, after proper treatment
- It sits in rough, chalky or flaking areas
- The area also sounds hollow when tapped, a sign of drummy render
- Stains are spread across much of the pool and no treatment holds
- The pool is also showing roughness, cracks or water loss
A worn surface is porous, so it picks up stains easily and keeps them. At that point you are fighting the surface, not the stain. Our guide on the signs your pool needs resurfacing goes through the rest, and our pool resurfacing page explains what a proper resurface involves.
Our advice: clean and balance first. If the stains still come back, book an inspection and we will tell you whether it is the water, the surface or both. If it is only the water, we will say so.
