What is the cheapest fix for metal pool stains?

Start with vitamin C, but only if you’ve identified the metal first. Iron stains (brown/rust) respond well to ascorbic acid; pour crushed vitamin C tablets into a sock and rub the stain—it fades almost immediately if it’s iron. If nothing happens, you’re dealing with copper or manganese, which need a sequestering agent from the pool store. Don’t raise chlorine before treating, as it sets metal stains permanently.