How do I remove metal stains from a pool?

Use ascorbic acid (vitamin C) — it’s cheap, safe on liners, and dissolves copper and iron stains in minutes. Drop crushed tablets directly on the stain or use a pool-specific product. Avoid chlorine shock, which makes stains worse, and don’t use muriatic acid alone. After the stain lifts, keep pH balanced and use a metal sequestrant to prevent recurrence.