A black stain shows up on the pool floor right after you add cal-hypo and your brain jumps straight to “drain it and acid wash.” We slow that moment down and explain what’s usually happening: copper that was already there, hiding as copper sulfate, getting oxidized by chlorine into black copper oxide. Once you understand that chemistry, you stop chasing the symptom and start fixing the cause. We also get practical about what actually works in pool stain removal and metal control. Ascor...