Quick Definition:

Black (also called blue-green) algae can appear in pools, often as small round dark spots at or near crevices, cracks, and shaded areas of the pool.

What is Black Algae in swimming pools?

Black algae, also called blue-green algae can form in swimming pools, and is evident by the growth of coin-sized black or blue-green spots.  These algae formations grow from cracks and crevices of cement or plaster-lined pools, and in shady areas of the pool.  Black algae occurs much less in vinyl-lined pools, as there are less imperfections in the pool walls and floor where the black algae can root.

The black algae forms layers, where the outermost layers protect the underlying layers, which in turn protect the algae roots.  Even if the outer layers are killed and scrubbed away, the lowest surviving layer will resume growth, making black algae difficult to completely eradicate.

How can you rid Black Algae in swimming pools?

Algaecides used in regular concentration in pools are generally not effective on black algae.  It is also resists chlorine, flourishing in chlorine levels considered normal for sanitation.  Directly scrubbing each black algae formation and then applying a sanitizer and algaecide directly has shown to be effective, but comes at significant cost in money and time to drain the pool to gain access to the algae growth.


  • Pool & Spa Water Chemistry, A Testing and Treatment Guide, Waterproof Edition, 2005. Taylor”

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