The vitamin C test is the fastest way to classify a pool stain:
Alternatively, use the chlorine test: rub granular pool shock directly on the stain. If it lightens: organic stain. If no reaction: metal or mineral.
| Stain Color/Appearance | Likely Type | Common Source |
|---|---|---|
| Green, brown, or black (under leaves or at waterline) | Organic | Leaves, algae, tannins, berries |
| Reddish-brown or rust-colored | Iron (metal) | Iron in fill water, corroded fittings |
| Blue, blue-green, or teal | Copper (metal) | Copper pipes, algaecide, heater |
| White, grey, rough/crusty deposit | Mineral/calcium | Hard water, high pH, high calcium |
| Dark spots with halo on plaster | Black algae | Algae with deep roots |
Organic stains (from leaves, algae, tannins from berries or wood) respond to chlorine:
Never use a metal scraper or steel wool on pool tile. Metal tools scratch the tile surface and accelerate future calcium buildup by creating microscopic grooves. Use only pumice stones or plastic scrapers specifically rated for pool tile.
Track when stains appeared, what you used, and how they responded. PoolLens helps you identify patterns — recurring metal stains indicate a water source or equipment issue that needs permanent fixing.
Open PoolLens Free →Do the vitamin C test: crush a vitamin C tablet and rub it on the stain for 30 seconds underwater. If it lightens, it is a metal stain. If there is no reaction, it is organic or mineral. Brown/green stains under leaves are usually organic. White crusty deposits are mineral/calcium.
Iron stains (reddish-brown) respond to ascorbic acid treatment. Lower chlorine to near zero, add 1–2 lbs ascorbic acid powder per 10,000 gallons, brush the stains, and follow with a metal sequestrant to prevent recurrence.
Copper stains (blue or blue-green) also respond to ascorbic acid treatment — same protocol as iron stains. Identify and address the copper source (corroded pipes, algaecide, heater) to prevent recurrence.
Light calcium scale responds to commercial tile cleaner with a pumice stone. Heavy calcium scale requires professional acid washing or bead blasting. Never use metal scrapers — they scratch tile and accelerate future buildup.
Stains recur when the source is not fixed. Metal stains return when the fill water has high metal content or corroded equipment keeps releasing metals. Organic stains return when leaves or debris are not regularly removed. Calcium stains return when pH or calcium hardness is chronically high.