How do I get stains out of my pool?

First figure out what kind of stain it is, because the wrong product sets it for good. Metal stains (iron, copper, manganese) show reddish brown, blue-green, or purple. Organic stains from leaves and algae look leaf-shaped or blobby. Hold a crushed vitamin C tablet on it for 30 seconds: if it lightens, it's metal, treat with ascorbic acid. If nothing changes, it's organic, treat with chlorine.

How do I do the vitamin C test on a pool stain?

Crush a vitamin C tablet and hold it against the stain for about 30 seconds. If the stain lightens or disappears under the tablet, it's a metal stain and comes out with ascorbic acid. If the stain doesn't change at all, it's organic and needs chlorine instead.

Why do I have to drop chlorine to zero before an ascorbic acid treatment?

Chlorine and ascorbic acid fight each other, so if you dose acid with chlorine still in the water you're just burning off product and money. Lower free chlorine to zero first, add ascorbic acid at roughly half a pound per 10,000 gallons, brush, and let it work before you slowly raise chlorine back over 24 to 48 hours.

Do I need a sequestrant after removing a metal stain?

Yes. Once the ascorbic acid lifts the metal off the surface, the metal is loose in the water and will redeposit. Add a metal sequestrant like HEDP or phosphonic to hold it in solution, and run one monthly if your fill water carries iron or copper. Keeping LSI in range also stops metals from precipitating back onto the plaster.

What if an organic stain won't fade after shocking?

Most organic stains fade within 24 hours once you raise free chlorine to SLAM level for your CYA and brush the area. If a stubborn one is still hanging on, add an enzyme cleaner on top of the chlorine to break down the organic load the chlorine alone isn't clearing.

How do I stop pool stains from coming back?

Chase the source, not just the stain. Run a metal sequestrant monthly if your fill water has iron or copper, pull leaves and organic debris out within 24 hours instead of letting them sit and bleed tannins, and keep your LSI in range so dissolved metals don't precipitate onto the surface in the first place.

Troubleshooting Pool Problems · 6 min read · By Matt Balog

Pool Stains: Metal vs. Organic and How to Remove Each

A diagnostic: color, shape, and location tell you what caused it and what removes it without draining.

By Matt Balog, Founder & Lead Pool Technician · Updated July 4, 2026 · 6 min read

Quick answer

Identify the category before you touch anything — metal stains (iron, copper, manganese) versus organic stains (leaves, berries, algae residue); the wrong product sets the stain permanently.
Color tells you the cause — reddish brown is iron, blue–green is copper, purple/black is manganese, leaf-shaped brown/black is organic tannins, and irregular green/brown blobs are dead algae.
The vitamin C test settles it in 30 seconds — hold a crushed tablet on the stain; if it lightens it's metal, if nothing changes it's organic.
Metal stains come out with ascorbic acid, not chlorine — drop free chlorine to zero, dose the acid, brush, then lock the metals up with a HEDP or phosphonic sequestrant.
Organic stains come out with chlorine, not acid — raise free chlorine to SLAM level for your CYA, brush, and most fade within 24 hours.
Fix the source or it comes back — sequester metals in fill water monthly, pull debris within 24 hours, and keep LSI in range so metals don't precipitate.

Pool stains fall into two categories: metal stains (iron, copper, manganese) and organic stains (leaves, berries, algae remnants). Getting the right category before treating is everything — the wrong product will set the stain permanently.

Diagnosing the stain

Color	Likely cause
Reddish brown	Iron
Blue-green	Copper
Purple/black	Manganese
Brown/black in leaf-shape	Organic (tannins)
Green/brown irregular blob	Dead algae residue

The vitamin C test

Hold a crushed vitamin C tablet against a stain for 30 seconds.

Stain lightens or disappears: Metal stain.
Stain unchanged: Organic stain.

Removing metal stains

Use an ascorbic acid treatment:

Lower free chlorine to 0 (chlorine and ascorbic acid fight — don't waste money).
Add ascorbic acid per package directions (typically 0.5 lb per 10,000 gal).
Brush the affected area.
After 30 minutes to a few hours (depending on stain age and metal load), stains should be lifting. Add a metal sequestrant (HEDP or phosphonic) to hold metals in solution.
Slowly raise FC back to normal over 24–48 hours.
Fix the source: filter municipal fill water, replace corroded heater components, or avoid adding iron-based shock.

Removing organic stains

Organic stains respond to chlorine, not ascorbic acid.

Raise FC to SLAM level for your CYA.
Brush the stain area.
Most organic stains fade within 24 hours. Persistent ones may need enzyme cleaner in addition to chlorine.

Preventing future stains

Use a metal sequestrant monthly if fill water has iron/copper.
Remove leaves and organic debris within 24 hours — don't let them sit.
Keep LSI in range to prevent metal precipitation.

Frequently asked questions

Metal and organic stains respond to opposite chemistry. Ascorbic acid lifts metal stains but does nothing for organic ones, and chlorine clears organic stains but fights ascorbic acid. Guess wrong and you can set the stain in the plaster permanently, so I run the vitamin C test before I touch anything.