Black algae needs a hard brush and heavy chlorine.
Scrub it hard with a stiff stainless steel brush and hit the pool with a strong chlorine shock. Then keep scrubbing.
Black algae looks like small black or dark blue-green spots on pool walls, usually in rough plaster or gunite. It feels slippery at first but gets a rubbery, waxy crust if left alone. The giveaway: brushing doesn’t instantly remove it. It’s way tougher than green algae because it digs roots into the surface.
Treatment is simple but requires patience:
- Brush aggressively with a stainless steel pool brush every day. You’re trying to break the waxy coating so chlorine can reach the roots.
- Raise free chlorine to shock level (consult your pool’s CYA level — generally 10-20 ppm for a few days). Don’t just toss in a bag and hope.
- Run the filter 24/7 and backwash regularly.
- If it’s really stubborn, apply a granular pool shock directly on the spots or use a phosphate remover to starve it.
Skip the fancy algaecides. Chlorine and elbow grease are cheaper and work better.
Don’t let it spread. One tiny spot can turn a wall into a polka dot nightmare in a week.