Which statement describes black algae in pools?

• A. Eradicates quickly with standard chlorine
• B. Roots extend into plaster or tile grout
• C. Grows only in metal surfaces
• D. Is a form of green algae

Answer: (B)

Black algae are a tough, embedded form of growth that doesn’t just sit on the surface. They form colonies with root-like filaments that extend into the pool surface, especially into porous plaster, grout, and small cracks. Those embedded roots anchor the colony and shield it from standard sanitizing, so ordinary chlorine struggles to reach and kill the entire organism. That’s why eradication requires physical brushing to scrub away the embedded mat, followed by thorough cleaning and maintaining high sanitizer levels to prevent a comeback, sometimes even resurfacing damaged areas. This understanding makes the statement about roots extending into plaster or tile grout the best description. The other ideas aren’t accurate: black algae aren’t eliminated quickly by standard chlorine, they don’t grow only on metal surfaces, and they are not simply a form of green algae.