Cal-Hypo and Black Pool Stains: Bob Lowry Explains

Clear blue water hides a lot of chemistry. In this conversation with industry legend Bob Lowry, we trace two common pain points that can rattle even seasoned pool techs: sudden black stains after dosing with calcium hypochlorite and the real health stakes when a pool slips to zero chlorine. The black stain scare can feel like a catastrophe, but it’s usually copper revealing itself, not a new problem. Copper sulfate can sit quietly on plaster, masked by the blue of the water. The instant cal hypo lands on it, oxidation shifts the compound to copper oxide, which is black and obvious. What changed isn’t the metal load, only its form—and that insight reshapes how we fix it.

Once you confirm a metal stain, you have options. Ascorbic acid can lift the stain from surfaces, but it doesn’t remove copper from the system. Without metal removal, the stain returns as soon as chlorine wipes out the reducer. That’s where sequestrants and chelants come in: they surround dissolved metal and keep it from plating onto surfaces. The catch is they degrade under UV, chlorine, ozone, non-chlorine shock, and AOP. Labels aren’t upsell theater; they’re reminders to re-dose because oxidizers chew through these molecules. The better move is to physically remove metals: use a CuLator bag in the skimmer for slower capture or run a dedicated pump through a metal pre-filter for faster reduction. Draining and refilling part of the pool works if source water is cleaner.

Application habits matter. Dropping cal hypo straight to the floor concentrates the reaction at a single point, making hidden copper show up as a stark black mark. Dissolve in a clean bucket and pour slowly around the perimeter or in front of returns so particles don’t settle. Even better, rethink cal hypo on known metal pools until metal levels are down. These simple changes reduce stain triggers and keep plaster safer. Small procedural improvements—stirring, distributing, and verifying flow—often save hours of remediation and grumpy client calls.

Algae management is a numbers game. Algae reproduce by cell division, doubling rapidly until a light dusting becomes a colony. By the time you can see it, a biofilm has likely formed—a clear polysaccharide dome that shields cells from sanitizer. That’s why brushing isn’t optional. Brushing breaks the slime layer and exposes algae to chlorine and algaecide. Ignoring small patches for a week makes the clean-up cost soar, demanding higher chlorine, longer filtration, and often floc or clarifier to recover clarity. The cheapest day to fix algae is the first day you notice it.

Health protection comes down to one thin shield: sanitizer residual. People bring roughly a hundred million bacteria into the water, and some species multiply far faster than algae. Without residual chlorine or an approved alternative, bather-to-bather disease transmission becomes likely, especially when swimmers get water in their mouths or introduce other contaminants. If you ever end up swimming where chlorine has dropped to zero, take a warm shower as soon as possible. Warmth helps reopen pores, and soap rinses organisms from the skin before they become a bigger problem.

For pool pros and owners, the larger lesson is simple: treat causes, not just symptoms. Metals must be removed or continuously sequestered with disciplined re-dosing. Cal hypo must be applied with intention. Algae must be brushed early, not after it blooms. And sanitizer residual isn’t a number to “check later”—it’s the barrier that keeps a fun swim from turning into an illness. Pair sound chemistry with good habits, and clear water stays both beautiful and safe.