How do I remove rust stains from a concrete pool without draining it?

For light surface stains, rub vitamin C tablets directly on the stain underwater — the ascorbic acid dissolves rust in seconds. For tougher stains, use oxalic acid powder dissolved in water, brush it on, let it sit a minute, then scrub and rinse. Avoid muriatic acid unless you’re experienced, as it can etch the surface. Always test on a small hidden spot first.