Black algae loves neglected chlorine levels and unbrushed walls.

Keep your chlorine between 1-3 ppm, brush the walls and floor at least once a week, and shock the pool every seven days during swim season.

Black algae is basically the cockroach of pool algae. It plants roots into concrete and forms a waxy protective layer that chlorine can’t penetrate on its own. That’s why you need a stiff stainless steel brush — the physical scrubbing breaks that barrier so chlorine can actually kill it.

Don’t let it start. Test your water weekly. Keep pH between 7.2-7.6. If you see tiny black spots, act that day. Brush aggressively and hit it with a 2x dose of shock. Skip the cheap algaecides — they rarely work on black algae.

A clean concrete pool is about routine, not emergency fixes.

Future you will thank you for the brushing habit.