What does high chlorine demand in a pool mean?

High chlorine demand means something is eating your chlorine, usually algae, heavy bather load (sweat, pee, sunscreen), leaves, or too much stabilizer blocking chlorine. Fix it by shocking the pool to 10–20 ppm free chlorine, brushing everything, and running the filter 24 hours. If chlorine still disappears fast, check for hidden algae in crevices, plumbing, or a dirty filter. Stop adding little bits of chlorine; you need enough to break through the demand.