Bob Lowry Explains the Cal Hypo Stain Mystery
Black stains after adding calcium hypochlorite (cal-hypo) can feel like a disaster, especially if you’re a pool service tech worried about a drain and acid wash. The key idea: the stain often existed before the shock went in, but it was invisible. Copper sulfate can sit on the floor or walls with a faint blue-green tint that blends into blue water, especially at depth. When strong chlorine hits that copper compound, it changes the chemistry by oxidizing it, converting copper sulfate into copper oxide. Copper oxide is black, so the “new” stain is frequently a hidden metal deposit revealed by oxidation, not a brand-new surface failure. Understanding pool staining chemistry helps you respond calmly and fix the actual cause. Many pros reach for ascorbic acid for stain removal, and it can work well to lift metal stains quickly. The catch is that ascorbic acid does not remove copper from the pool water. It can reduce and loosen the stain, but the metal remains in circulation, wait...