Brown or Green Stains in a Pool

Stain diagnosis is a two-family detective story, and the families hate each other’s medicine. Metal stains answer to ascorbic chemistry and sequestrants; organic stains answer to chlorine and brushing. Applying the organic cure to a metal stain wastes a week, and the reverse wastes the same week with more confidence. The vitamin C tablet test exists precisely because eyes cannot separate a rust shadow from a tannin shadow reliably.

Symptoms

• Brown or rust-colored patches on floor and steps
• Green-black smudges that brush like growth, or that do not
• Stains appearing after a fill, a treatment or leaf load
• Ring stains under where leaves or metal objects sat
• Shadows that remain after the water is balanced and clear

Sources matter as much as stains. Iron rides in on fill water in parts of Lee County, copper leaches from a heater exchanger being eaten by low-pH water, and both keep arriving until the source is named. A pro’s stain visit is therefore half chemistry panel and half plumbing inspection, and the written finding, not the product, is the deliverable.

Stains reward detectives. The vitamin C touch test and the chlorine touch test, performed on one spot each, divide the organic world from the metal world in a minute, and the two worlds have opposite cures. Guessing wrong is worse than waiting, because metal treatments scatter organic stains and chlorine deepens metal ones. In a mineral county the metals usually arrive through fill water, which makes the stain a water report as much as a blemish. Photograph stains beside a coin for scale and date the picture; stain behavior across two weeks is evidence, because organic shadows fade slowly under steady chlorine while fresh metal deposits often darken after shocking, and that difference confirms the touch tests.

Common Causes

Cause	How likely	What a homeowner can check
Iron or copper in fill water or from corroding parts	Common	Hold a vitamin C tablet on the stain; metal stains lighten visibly
Organic tannins from leaves and debris	Very common	Recall debris history at the exact stained spots
Algae colonies darkening the surface	Common	Brush hard; growth smears, mineral stain does not

Questions and Answers

What does the vitamin C test prove?

Ascorbic acid lifts metal stains on contact. A spot that lightens under the tablet in a minute is metal; no change points to organic staining.

Can stains be removed without draining?

Most can, with in-water treatments matched to the family, plus source control. Severe or etched cases sometimes justify a drained acid treatment by a pro.

Why did stains appear right after shocking?

Shocking can push dissolved metals out of solution onto the surface, revealing a metal load that was invisible at lower oxidizer levels. The shock exposed the problem; it did not create the metals.

Sources

• University of Florida IFAS Extension publications
• CDC Healthy Swimming guidance on pool water quality