Pool Stains: The Vitamin C Test and What Each Color Means
Updated September 2026
Quick answer: Every stain is either organic (leaves, algae, berries — browns and greens) or metal (iron's rust-brown, copper's blue-green-black, manganese's purple-black). Sixty seconds settles it: hold a vitamin C tablet on the stain — if it lightens dramatically, it's metal (iron especially). Hold a trichlor tab on it — lightening means organic. Organic stains die by shock-and-brush; metal stains need ascorbic-acid treatment plus a sequestrant, and pouring more chlorine on a metal stain actually sets it darker.
The 60-second classification
Test one — vitamin C: press a plain ascorbic-acid tablet against the stain for 30 seconds. Dramatic lightening = metal stain, almost certainly iron. Test two — chlorine: hold a trichlor tablet (in a holder or glove) on the spot; fading = organic. Neither moving it strongly suggests copper or manganese — the metal family's stubborn cousins, which the ascorbic treatment still addresses.
The color decoder as a starting hint: green-brown that brushes pale = organic (leaves, tannins, early algae); rust-orange-brown = iron (well water, corroding hardware); blue-green to black-green = copper (algaecides, heater corrosion, source water); purple-black = manganese (some well supplies). Location hints too: organic stains sit under where debris rests; metal staining spreads broadly or follows return flow.
Removing each kind
Organic: shock the pool, brush the stains daily, and let elevated chlorine bleach out what the debris left behind — most fade within days. Stubborn tannin shadows (oak, walnut) may need a second round or an enzyme assist, and the prevention is boring: skim and vacuum before debris marinates.
Metal: lower chlorine to near zero (metal removal and chlorine oppose each other), lower pH to ~7.2, then dose ascorbic acid — roughly half a pound per 10,000 gallons, circulated — and watch decades-looking rust vanish in an hour. Immediately follow with a metal sequestrant so the now-dissolved metal doesn't restain when chlorine returns, then rebuild chlorine gently over days while keeping pH at the low end.
Keeping metal stains gone
Ascorbic acid dissolves the stain, but the metal is still in the water — sequestrant binds it in suspension, and sequestrants degrade, so quarterly maintenance doses are part of the deal for pools with metal-bearing fill water. Filling through an inline metal-trap cartridge cuts what comes in; a CuLator-style absorber in the skimmer removes what's already there over weeks.
Find the source or repeat the ritual: well water high in iron, copper algaecides (retire them permanently — polyquat instead), aggressive low-pH water leaching heater copper, or corroding ladder anchors. One source identified is worth three treatments — and if the heater is the donor, fixing water balance protects a four-figure appliance along with the plaster.
FAQ
How do I know if a pool stain is metal or organic?+
Hold a vitamin C tablet on it: dramatic lightening = metal (usually iron). Hold a chlorine tablet on it: fading = organic. The two 30-second tests classify virtually every stain before you buy anything.
How do I remove metal stains from my pool?+
Drop chlorine near zero, pH to ~7.2, dose ascorbic acid (~½ lb per 10,000 gal) — stains lift within an hour — then add sequestrant immediately and rebuild chlorine slowly. Skipping the sequestrant invites the stain straight back.
Why did my pool stain get worse after shocking?+
It's a metal stain: chlorine oxidizes dissolved iron and copper out of solution and onto surfaces, setting them darker. Metal stains need the ascorbic-acid path, not more chlorine.