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.
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.
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.
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.
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.
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.
Want a pro to handle this?
Our CPO-certified techs run this exact playbook on every weekly service visit.