Black Algae

Black algae earns its reputation honestly. The visible spot is a cap; beneath it, root-like structures work into the plaster, sheltered from sanitizer by the colony’s own outer layer. Wiping chlorine across the surface kills the cap and leaves the root, and the spot returns like a weed pulled without its base. Effective treatment is mechanical first: a stiff brush, steel on plaster where the surface allows, breaks the cap so chemistry can reach the root.

Symptoms

• Dark blue-green to black spots on walls and floor
• Spots concentrated in shady, low-circulation corners
• A slimy cap that smears when brushed hard
• Spots returning in the same places after treatment
• Rough plaster or grout at the center of each colony

The second half of treatment is geography. Colonies cluster where circulation is lazy, behind ladders, in corners, along the deep-end floor seam, so those zones deserve brushing attention long after the spots fade. Owners who treat black algae once and win usually changed a habit too, because the colony needed a chlorine gap to start, and closing that gap is the actual cure.

Black algae punishes cosmetic thinking. Scrubbing the heads off colonies while the roots keep their shelter produces a pool that looks cured on Saturday and spotted again by the next month, which is why experienced hands describe the treatment as dentistry rather than cleaning. The other lesson is prevention geometry: colonies start where circulation and the brush reach last, so a monthly pass over steps, benches and light niches is worth more than any product on the shelf. Prevention has a geography worth memorizing: the shaded wall, the step edges and the corners behind the ladder are the neighborhoods this organism chooses first, so the monthly brush route should start there instead of ending there when energy runs out.

Common Causes

Cause	How likely	What a homeowner can check
Colonies established during a low-chlorine period	Very common	Map where spots sit; returns in identical spots mark rooted colonies
Rough or aging plaster giving roots a grip	Common on older pools	Feel the surface texture at a spot with a brush handle
Dead circulation zones	Common	Watch where debris settles; still corners collect both

Questions and Answers

Is black algae dangerous?

The algae itself is mainly a harbor: its rough colonies shelter bacteria and make surfaces slick. The greater issue is that it signals sanitizer gaps and damages plaster texture over time.

Can black algae stain the pool permanently?

Long-standing colonies can leave shadow marks in plaster after the growth dies. Early treatment limits both the roots and the staining.

Why does black algae keep returning to the same spots?

Roots left in the plaster regrow. Complete treatment breaks the cap, exposes the root to chemistry and repeats until test brushing shows no regrowth.

Sources

• CDC Healthy Swimming guidance on pool water quality