How do I get rid of black algae in my pool?

You need a copper-based algaecide and a stiff brush. Regular chlorine won’t cut it. Brush aggressively with a stainless steel or nylon brush to physically disrupt the roots, then apply copper algaecide, superchlorinate, and repeat daily for about a week. Prevention is easier—keep chlorine and pH balanced, brush weekly, and attack any black spot immediately.