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, waiting for the next oxidizing event. As soon as you reintroduce chlorine, you can burn off the ascorbic acid and the copper can redeposit, bringing the stain right back. That’s where sequestrants and chelating agents come in: they bind or “surround” dissolved metals to keep them from precipitating and staining. They are not magic filters, though, because they degrade from sunlight (photodegradation) and from oxidizers like chlorine, non-chlorine shock, UV systems, ozone, and AOP. If you choose a sequestrant strategy, consistent re-dosing based on the label is maintenance, not upselling.  

If your goal is a lasting fix, removing metals is stronger than endlessly managing them. Practical options include products designed to physically capture metals, such as a Q-Lator placed in the skimmer or used in a higher-flow setup. In a skimmer, removal can take a week or more, while pushing water through a dedicated prefilter with a separate pump can lower the metal level much faster. The other dependable approach is partial drain and refill to dilute copper concentration, especially when source water and equipment won’t keep reintroducing metals. Prevention matters too: avoid letting cal-hypo granules sit on the pool floor. Pre-dissolve in a bucket and broadcast evenly or feed in front of a return so it dissolves in suspension rather than bleaching and reacting in one concentrated spot.  

The episode also tackles two “what if” scenarios every pool owner and pool technician should take seriously: algae and zero chlorine. Algae multiplies by cell division, so a small visible patch can explode into a major bloom within days, and once mature it secretes a polysaccharide dome that shields it from chlorine. That’s why brushing is not optional; it breaks the protective layer so sanitizer can do its job, and brushing before vacuuming improves removal. Finally, swimming in a pool with no chlorine residual increases the chance of bather-to-bather disease transmission because bacteria multiply fast and bathers introduce large microbial loads. If you’re unsure about water quality, a warm shower after swimming can help rinse contaminants before pores close, reducing the chance of irritation or illness. These pool maintenance tips connect to one principle: consistent chemistry and physical cleaning prevent expensive problems later.