Black algae is the most serious and difficult algae problem a pool tech can encounter. Unlike green algae which responds reliably to a properly executed SLAM, black algae often requires weeks of aggressive treatment — and in well-established cases in older plaster pools, it may never be truly eradicated without replastering. Understanding why it is so different from other algae is the starting point for any effective treatment plan.
Despite the common name, black algae is Cyanobacteria — photosynthetic bacteria, not true algae. This matters because its cellular structure and survival mechanisms differ significantly from Chlorophyta (green algae) or Chrysophyta (mustard algae):
| Characteristic | Black Algae | Manganese Stain | Copper Stain |
|---|---|---|---|
| Texture | Raised, bumpy, rough — "cauliflower" appearance | Flat, smooth | Flat, smooth |
| Color | Dark green-black, blue-green tinge | Purple-black, brown | Green, teal, black |
| Location | Walls, especially shaded and rough spots | Widespread, often waterline | Widespread or near returns |
| Vitamin C tablet test | No reaction | Lightens or disappears | No reaction (different stain remover needed) |
| In vinyl pools? | Extremely rare | Yes | Yes |
Pool owners (and some techs) try to treat black algae by shocking the pool to 10–20 ppm and waiting. This approach kills the visible surface cells but does nothing to the subsurface roots. Within days, the colony regenerates from the protected root system and the black spots reappear.
Effective treatment requires physical disruption of the protective slime layer before chlorine application. Without mechanical disruption, chlorine concentrations that would be hazardous to humans still cannot penetrate the biofilm fast enough to kill the underlying colony.
This is the most important step. Using a stainless steel brush specifically (not nylon — it cannot penetrate the slime), scrub every black algae spot with significant force. The goal is to physically break through the protective biofilm and expose living cells to the water. Each spot should be scrubbed until the surface looks lighter or scratched.
Immediately after brushing each spot, rub or hold a trichlor tablet (or a broken piece of one) directly on the exposed algae for 30–60 seconds. The concentrated chlorine at the contact point creates a much higher local HOCl concentration than can be achieved through the bulk water. This direct application is critical for penetrating into the root zone.
After direct spot treatment, raise the entire pool to SLAM FC level based on CYA. Use liquid chlorine. Maintain SLAM level for the entire treatment period — check every 4 hours and redose.
Add copper algaecide at treatment dose per label. Simultaneously add sequestrant at the recommended dose. Copper ions provide a second mechanism of action against the cyanobacteria — disrupting their metalloenzymes.
Brush and spot-treat with trichlor tablets every single day until no black spots are visible. Each brushing disrupts new surface growth and re-exposes the root zone to chlorine. Do not skip days.
In plaster pools where black algae has been established for more than one season, the roots can penetrate 3–5 mm into the plaster surface. Chemical treatment in these cases may suppress growth but not eradicate it — the colony retreats deeper into the plaster and returns within weeks of any reduction in chlorine.
Recommend replastering when:
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Open PoolLens Free →Black algae (Cyanobacteria) forms thick protective coatings of gelatinous slime that prevent chlorine penetration, and its roots penetrate into plaster and concrete, making surface-level treatment ineffective. Even high chlorine levels kill only the visible surface layer while the protected root system survives to regenerate.
Black algae treatment requires: (1) Scrubbing with a stainless steel brush to break through the protective slime layer, (2) Applying trichlor tablets directly to the scrubbed spots, (3) Maintaining SLAM-level FC, (4) Copper algaecide treatment. Repeat brushing daily. In plaster pools with deep root penetration, full eradication may require replastering.
Black algae has a characteristic rough, raised, bumpy texture — almost like small cauliflower heads on the plaster. Manganese staining is flat and smooth. Rub a vitamin C tablet on the dark area: manganese stains will lighten or disappear; black algae will not respond to ascorbic acid.
In some cases yes — with aggressive stainless steel brushing, sustained high chlorine, and copper algaecide. However, in plaster pools with well-established black algae that has penetrated deep into the surface, chemical treatment often only suppresses rather than eliminates the problem, and replastering may be required for a permanent fix.