Water chemistry
Black algae
Black algae shows up as dark spots with deep roots, often in plaster, and needs hard brushing and treatment.
Black algae is the hardest type to kill. The dark spots grow roots into plaster, grout, and rough surfaces, and a protective layer shields them from chlorine.
Treatment means scrubbing each spot with a stiff stainless steel brush on plaster, then applying chlorine tablets or a targeted product straight onto the spot, and keeping free chlorine high. Avoid steel brushes on vinyl or fiberglass. Spots often return if the roots survive. For stubborn cases, see a pro. Treat cyanuric acid and pH levels too.
Related terms
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