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How to get rid of blackspot algae in your swimming pool
Understand black algae.
Like any alga species, black algae (which are really blue-green algae that look black) will grow quickly in warm water on a sunny day. Black algae are chlorine-resistant, because they have a protective outer layer that makes regular sanitization ineffective.
While algae aren't harmful, their presence can be a sign that other bacteria are present. Other alga species that are common in pools are green algae, mustard or yellow algae, and pink algae, which is actually a bacterium.