Black Algae
Black algae is a stubborn, deeply rooted algae species that forms black or dark blue-green spots on pool plaster surfaces.
Definition
Black algae is a stubborn, deeply rooted algae species that forms black or dark blue-green spots on pool plaster surfaces.
Typical Values: Treatment: 20–30 ppm FC maintained for 3–5 days; requires vigorous brushing
In Plain Language
Despite its name, black algae is a cyanobacterium rather than a true algae. It establishes a protective outer layer (sheath) that resists normal chlorine levels. Treatment requires physically scrubbing the spots aggressively with a stiff brush to break the protective layer, then maintaining very high FC levels (20–30 ppm) for several days. Black algae often recurs if not fully treated.
Why It Matters
Black algae is the hardest pool algae to eliminate and often indicates a long-standing chlorine deficit.
Typical Values
Treatment: 20–30 ppm FC maintained for 3–5 days; requires vigorous brushing
Last reviewed: 2026-06-01