Brushing and shocking are your only real weapons.

Keep your chlorine levels high and brush the walls at least once a week.

Black algae isn’t like green algae. It has a waxy protective layer and actually roots into the plaster or gunite. Chlorine alone can’t penetrate that barrier if the algae has already started forming. Physical brushing breaks that layer, allowing the sanitizer to do its job.

The real trick is prevention—never let the chlorine drop below 2-3 ppm, and shock the pool weekly during warm months. Some people add a phosphate remover or a pool-grade algaecide as backup, but those are insurance, not substitutes.

If you see black spots, scrub them hard with a stainless steel brush (nylon won’t cut it for black algae) and hit the spot with a chlorine tablet or a concentrated shock treatment directly. But again: regular brushing is the only thing that truly keeps it from coming back.

Black algae is stubborn as hell once it takes hold. You do not want to be the person trying to remove it mid-summer.