| Stain Color | Texture | Most Likely Type | Common Source |
|---|---|---|---|
| Reddish-brown, rust | Flat, smooth | Iron (metal) | Fill water, corroded fittings |
| Blue-green, teal | Flat, smooth | Copper (metal) | Copper pipes, algaecide, heater |
| Green or brown (under leaves) | Flat or slight relief | Organic (tannins/algae) | Leaves, debris, algae |
| Black spots with lighter halo | Slightly raised | Black algae | Cyanobacteria |
| White, grey, crusty | Rough, raised | Calcium scale | High pH + calcium hardness |
| Purple-black | Flat | Manganese (metal) | Well water or municipal supply |
Iron stains are among the most common pool stains. They appear as rust-colored marks on plaster, vinyl, or fiberglass, often in patterns that follow water flow or settle near the main drain and return jets.
Iron sources in pools:
Iron stains worsen when chlorine levels spike — chlorine oxidizes dissolved iron, causing it to precipitate and deposit on surfaces. This is why iron stains often appear or worsen immediately after shock treatment.
Copper stains produce unmistakable blue-green or teal marks, sometimes with a turquoise hue. Copper is a well-known algaecide and is naturally algae-resistant, which is why copper staining is sometimes confused with algae — the colors overlap.
Copper sources:
Low pH (below 7.0) is the primary driver of copper corrosion — acidic water aggressively dissolves copper from pipes and heat exchangers. Pools with persistent low pH develop ongoing copper release and staining.
Organic stains result from plant matter and biological material that settles on pool surfaces and leaves pigmented deposits behind. Unlike metal stains, organic stains are not caused by dissolved minerals but by the physical contact of organic matter with pool surfaces.
Common organic stain causes:
Organic stains respond to chlorine shock and brushing — the oxidizing action of chlorine breaks down the organic pigments. They do not respond to ascorbic acid treatment (which distinguishes them from metal stains in the vitamin C test).
Calcium carbonate scale forms when oversaturated water deposits calcium on pool surfaces. It appears as white, grey, or off-white rough deposits, most commonly at the waterline where evaporation concentrates minerals. Unlike most stains, calcium deposits have a three-dimensional quality — they build up on the surface rather than staining into it.
Calcium deposits are caused by:
The vitamin C test is the fastest way to classify any pool stain: crush a vitamin C (ascorbic acid) tablet and hold the powder directly against the stain underwater for 30 seconds. If the stain lightens, it is a metal stain (treat with ascorbic acid). If there is no reaction, it is organic (treat with chlorine) or mineral (treat with acid or pumice).
PoolLens logs metal readings, pH, and calcium hardness — helping you spot the chemistry conditions that lead to staining before they become permanent marks on your pool surfaces.
Open PoolLens Free →Reddish-brown stains are iron. Sources: fill water with high iron, corroded steel fittings or rebar. Chlorine oxidizes dissolved iron causing it to precipitate on surfaces. Treatment: ascorbic acid dissolves the stain, followed by a metal sequestrant to prevent recurrence.
Blue-green or teal stains are copper. Sources: corroded copper pipes or heater, copper algaecide applied at too high a dose or low pH, copper ionizer over-releasing. Low pH accelerates copper corrosion. Treatment: ascorbic acid removes the stain; address the copper source to prevent return.
Black spots with a lighter halo on plaster are almost always black algae — living organisms with roots that penetrate the surface. True mineral staining from manganese can also appear dark. The vitamin C test distinguishes them: algae won't react; manganese stains lighten with ascorbic acid.
Metal stains: test fill water for metals, add sequestrant monthly, avoid copper algaecide, inspect equipment. Organic stains: remove debris promptly, maintain adequate chlorine, brush regularly. Calcium stains: keep pH at 7.2–7.4, calcium at 200–400 ppm, add scale inhibitor monthly.
Stains recur when the source isn't fixed. Metal stains return when fill water has high metal content or corroded equipment keeps releasing metals. Organic stains return when debris isn't removed consistently. Calcium stains return when pH or calcium hardness is chronically high. Fix the source, not just the stain.