Black algae means you’ve got a dead spot in your pool.

It’s caused by poor circulation and low chlorine levels, usually in shady corners or on rough plaster surfaces where the water doesn’t move much. Black algae roots into the surface, so regular brushing won’t cut it.

To kill it, you need to brush aggressively with a stainless steel brush (nylon won’t touch it), then double- or triple-shock the pool. Keep the chlorine high for a few days and brush every day to break up the roots. After it turns gray or white, vacuum to waste and scrub again.

Prevention is about circulation—run your pump longer, especially during hot weather, and keep an eye on those shady spots.

Future You will thank you for not letting it spread.