How do I know if my pool stain is metal or organic?

Hold a 500 mg vitamin C tablet against the stain for 30 seconds with a brush. If the stain lifts and leaves a lighter spot, it is metal (iron, copper, manganese). If nothing happens, it is organic and chlorine will remove it.

What is the best way to remove iron stains from a pool?

Drop free chlorine below 1 ppm, lower pH to 7.2, add 1 lb of ascorbic acid per 10,000 gallons, brush the stains, then add a HEDP-based metal sequestrant to keep the iron in solution. Bring chlorine back to 3 ppm slowly over 3 to 4 days.

Why did my pool stain come back after I removed it?

You almost certainly skipped the metal sequestrant, or you stopped using one. The moment chlorine rises, dissolved metals re-oxidize and redeposit onto the surface. Add a sequestrant per label and dose monthly during swim season.

Can I use a pumice stone in a vinyl pool?

No. Pumice stones, stainless brushes, and acid washing will permanently damage a vinyl liner. Use nylon brushes only and stick to liner-safe products like ascorbic acid and liquid chlorine.

How do I tell black algae from a manganese stain?

Scrub it hard with a stiff brush. If it lifts (even partially) and feels slightly waxy, it is black algae. If it does not budge no matter how hard you scrub, it is a manganese metal stain and needs ascorbic acid treatment, not chlorine.

What dosage of ascorbic acid removes pool stains?

1 pound of ascorbic acid per 10,000 gallons, broadcast directly onto the stain or evenly across the pool. Brush within a few minutes to spread the effect. Always neutralize chlorine first or the ascorbic acid is consumed before it can work.

Will shocking the pool remove all stains?

Only organic stains (algae, leaves, pollen, pink slime). Shocking a metal stain will lock it in permanently and make it much harder to remove. Always run the vitamin C test before you shock.

How can I prevent stains if I fill my pool from a well?

Use a hose-end metal filter (PoolMaster Pre-Filter or Culator) every time you add water. Add a metal sequestrant to the pool monthly. Keep pH between 7.4 and 7.6 — high pH precipitates iron fast.

Pool maintenance

Pool stains: identify and remove every type

By Derek Bowen, founder of Pool Rental Near Me and author of 7 books on pool hosting · Updated May 31, 2026

Pool stain color tells you what caused it. Green/brown is metal (iron, copper). Blue/black is also copper or manganese. Pink is bacteria. Black is algae. Test a hidden spot with a vitamin C tablet (metal) or chlorine puck (organic) before treating the whole pool.

2 min read · Updated May 31, 2026

Identify before you treat

Touching the wrong chemical to the wrong stain can permanently set it. Always identify first.

Color	Likely cause	Quick test
Green	Iron from well water or fertilizer	Vitamin C tablet held against stain (lifts in 30 sec)
Brown / red-brown	Iron or organic	Vitamin C test
Blue / blue-green	Copper from algaecide or heater	Vitamin C
Black	Black algae OR manganese	Brush hard. If it lifts, algae. If not, metal
Pink	Pink slime bacteria	Lifts with chlorine
Yellow	Mustard algae or pollen	Brush, then chlorine

The vitamin C test

Drop a 500 mg vitamin C tablet on a stain. Hold it there for 30 seconds with a brush. If the stain lifts (lighter spot under the tablet), it is metal. If it does not, it is organic and chlorine will fix it.

This is the single most useful test in stain removal.

Removing metal stains

Drop chlorine to under 1 ppm (sun, or use a chlorine neutralizer)
Add ascorbic acid (1 lb per 10,000 gallons) directly to the pool
Brush the stains. They lift in minutes
Add a metal sequestrant (HEDP-based) per label to hold the iron in solution
Run the pump 24 hours
Slowly bring chlorine back to 3 ppm over 3 to 4 days

If you skip the sequestrant, the metal will redeposit as soon as chlorine returns and the stain comes back.

Removing organic stains

For organic stains (algae, leaves, pollen), chlorine is the answer. Brush the stain, raise free chlorine to shock level, hold for 24 hours.

For tough black algae, scrub with a pumice stone (plaster pools only, never vinyl) or stainless brush, then drop a chlorine tablet directly on the spot for an hour.

Pink slime

Pink slime is a bacterial biofilm that resists normal chlorine. Treatment:

Clean the filter completely (it lives there too)
Shock to 30 ppm chlorine
Hold shock for 24 to 48 hours
Scrub all surfaces
Run pump continuously

Stains in vinyl liner pools

Be careful. Pumice stone, stainless brush, and acid washing will ruin a vinyl liner. Use only nylon brushes and approved liner-safe products. For deep stains, sometimes the only fix is a new liner.

Prevent stains from coming back

Add a metal sequestrant once a season (iron, copper)
Pre-filter well water with a hose-end metal filter
Avoid copper-based algaecides
Keep pH in range (high pH precipitates metals)
Keep an eye on heater corrosion (a leaking heater dumps copper)

When to call a pro

Call for full pool acid washing (plaster only, nasty job, do not DIY). Call for stains that come back within a week of treatment (you have a metal source you have not found). Call before stripping a vinyl liner with anything aggressive.

Frequently asked questions

How do I know if my pool stain is metal or organic?: Hold a 500 mg vitamin C tablet against the stain for 30 seconds with a brush. If the stain lifts and leaves a lighter spot, it is metal (iron, copper, manganese). If nothing happens, it is organic and chlorine will remove it.
What is the best way to remove iron stains from a pool?: Drop free chlorine below 1 ppm, lower pH to 7.2, add 1 lb of ascorbic acid per 10,000 gallons, brush the stains, then add a HEDP-based metal sequestrant to keep the iron in solution. Bring chlorine back to 3 ppm slowly over 3 to 4 days.
Why did my pool stain come back after I removed it?: You almost certainly skipped the metal sequestrant, or you stopped using one. The moment chlorine rises, dissolved metals re-oxidize and redeposit onto the surface. Add a sequestrant per label and dose monthly during swim season.
Can I use a pumice stone in a vinyl pool?: No. Pumice stones, stainless brushes, and acid washing will permanently damage a vinyl liner. Use nylon brushes only and stick to liner-safe products like ascorbic acid and liquid chlorine.
How do I tell black algae from a manganese stain?: Scrub it hard with a stiff brush. If it lifts (even partially) and feels slightly waxy, it is black algae. If it does not budge no matter how hard you scrub, it is a manganese metal stain and needs ascorbic acid treatment, not chlorine.
What dosage of ascorbic acid removes pool stains?: 1 pound of ascorbic acid per 10,000 gallons, broadcast directly onto the stain or evenly across the pool. Brush within a few minutes to spread the effect. Always neutralize chlorine first or the ascorbic acid is consumed before it can work.
Will shocking the pool remove all stains?: Only organic stains (algae, leaves, pollen, pink slime). Shocking a metal stain will lock it in permanently and make it much harder to remove. Always run the vitamin C test before you shock.
How can I prevent stains if I fill my pool from a well?: Use a hose-end metal filter (PoolMaster Pre-Filter or Culator) every time you add water. Add a metal sequestrant to the pool monthly. Keep pH between 7.4 and 7.6 — high pH precipitates iron fast.

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Written by the PRNM team

Pool Rental Near Me is the peer-to-peer pool rental marketplace America loves — connecting pool owners with guests for hourly rentals across the US. Our editorial team works with hosts and licensed pool pros to keep these guides current.