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, stains should be gone. 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.