How do I get rid of black algae on plaster pool surfaces?

Scrub the plaster with a stiff brush every few days and keep your chlorine and pH in the right range. Black algae forms a protective layer that regular chlorine can’t easily penetrate, so brushing breaks up the biofilm before it hardens. If it takes hold, you’ll need to shock and scrub repeatedly. Prevention is easier than cure—this is not a set-it-and-forget-it situation.