How do I remove stains from pool plaster without damaging it?

Start with a stain ID kit to identify the stain type. For metal stains (iron, copper, manganese), turn off the pump, broadcast powdered ascorbic acid directly over the stain, wait 15 minutes, then brush. For organic stains, use a good shock treatment or stain-specific enzyme. Never use copper-based algaecide if you already have copper stains.