Ascorbic acid works for metal pool stains.

Use vitamin C (ascorbic acid) — it’s cheap, safe on liners, and dissolves copper and iron stains without damaging anything. Drop a few crushed tablets directly on the stain or use a pool-specific ascorbic acid product. It works in minutes.

Avoid chlorine shock. Chlorine makes metal stains worse by oxidizing them. If you’ve already shocked, you’ll need to wait for chlorine levels to drop before treating. Also, don’t use muriatic acid alone — it can damage the liner if not diluted properly. Ascorbic acid is the safe bet.

After the stain lifts, you’ll want to prevent recurrence by keeping your pH balanced and using a metal sequestrant (like a chelating agent). Metals usually enter from fill water or algaecides, so test your source water if it’s a repeat problem.

Future You will thank you for not scrubbing with pool-store snake oil.