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The three stain families
Metals (iron browns, copper blue-greens) arrive dissolved and invisible, then oxidize onto surfaces — often right after a shock, which is the oxidizing event. Organics(tannin browns, algae greens) are stains with a biography: a leaf that sat all winter, a bloom that got killed but not scrubbed.Scale (white, rough) isn't dirt at all — it's your water precipitating calcium, a chemistry setting, not a cleaning problem.
The two-tablet diagnosis
A crushed vitamin C tablet held on the stain lightens metals within a minute (ascorbic acid re-dissolves them). A chlorine source held on the stain bleaches organics. Thirty seconds each, under $2, and it tells you which of two opposite treatments to buy — ascorbic-acid based stain remover plus sequestrant for metals, versus brushing and chlorine for organics.
Why stains reappear
Treatments lift the stain but not the cause. Metal stains return until the metal source is found (fill water, corroding heater core at chronic low pH) and a sequestrant is maintained. Scale returns until the saturation index is brought near zero. Organic stains return exactly as often as leaves are left to sit — the one fix that's pure habit.
Frequently asked questions
How do I tell what kind of pool stain I have?
Two cheap tests: hold a vitamin C tablet against the stain for 30 seconds — if it lightens, it's metal (iron/copper). Put a chlorine tablet (in a sock) on it — if that lightens it, it's organic (leaves, algae remnants). Neither budging suggests scale or a dye stain.
What causes brown stains in a pool?
Usually iron — from well water, corroding equipment, or fertilizer overspray. Greenish-black on surfaces can be copper (algaecides, heater corrosion at low pH) or organic. The vitamin C test separates them in half a minute.
Will shock remove pool stains?
Organic stains, often yes. Metal stains, no — high chlorine sets metal stains harder. That's why identifying before treating matters: the standard fix for one type worsens the other.
How do I keep stains from coming back?
Metals: find the source (test fill water, watch pH so it doesn't dissolve heater copper) and use a sequestrant during and after treatment. Organics: keep chlorine at target and debris skimmed. Scale: manage your saturation index — see the LSI calculator.