Black algae needs scrubbing, not chemicals alone.

You can’t just pour chemicals on it and hope. Black algae roots into the pores of concrete — it’s not floating around. The only way to kill it is to physically break those roots loose.

Get a stainless steel brush (nylon won’t cut it). Scrub the spots until you see the black cloud dissipate. Then shock the pool hard — triple your normal chlorine level. Keep scrubbing every day for a week. The algae will keep trying to come back until you’ve scrubbed the roots out completely.

Prevention is easier: brush your walls weekly, keep chlorine in range, and don’t let phosphates build up. Spot a dark patch early? Hit it with a high-chlorine puck directly on the spot for a day.

It’s a battle of persistence, not a magic bottle.