What is the best way to remove rust stains from pool concrete?

Oxalic acid works best for pool concrete rust stains. It lifts rust without eating the surface, unlike muriatic or phosphoric acid which are more aggressive and can mess with water balance. Wear gloves and eye protection, dilute per label (usually 1:10 with water), brush on, let sit 5–10 minutes, scrub, and rinse thoroughly with the pump running. For deep stains, repeat or use a poultice overnight. Use pool-specific oxalic acid, not hardware store versions, to avoid unknown additives.