What does black algae in my pool mean and how do I get rid of it?

Black algae means you’ve got a dead spot in your pool caused by poor circulation and low chlorine levels, usually in shady corners or on rough plaster. To kill it, brush aggressively with a stainless steel brush, then double- or triple-shock the pool and keep chlorine high for a few days while brushing daily. After it turns gray or white, vacuum to waste and scrub again. Prevent it by running your pump longer, especially in hot weather, and watching those shady spots.