Black algae needs chlorine shock and a wire brush

Raise your chlorine level and brush aggressively with a stainless steel brush until it’s gone.

Black algae (technically cyanobacteria) has a waxy coating that shrugs off normal chlorine levels and even most algaecides. The only way to kill it is to physically break that coating so chlorine can get in. That means brushing hard, not just a once-over with a nylon brush. Stainless steel bristles are non-negotiable.

Shock