High chlorine demand means something is eating your chlorine.

Yes, and it’s probably gross.

High chlorine demand happens when your chlorine gets used up faster than you can add it. The usual suspects: algae blooms, heavy bather load (sweat, pee, sunscreen), leaves, or stabilizer (cyanuric acid) that’s too high and actually blocks chlorine from working. Sometimes it’s just a ton of organic debris sitting in the water.

Fix it by shocking the pool hard—like 10–20 ppm free chlorine—and brushing everything. Run the filter 24 hours. If the chlorine still disappears in a few hours, you’ve got a persistent source: check for hidden algae in crevices, plumbing, or a filter that hasn’t been cleaned in months. Stop adding little bits of chlorine; you need enough to break through the demand.

Stop dumping chlorine without cleaning first.