Black algae is the hardest algae to kill. It roots into plaster, grows a protective waxy layer, and ignores normal chlorine levels. If you see black spots the size of dimes or quarters on your pool surface — especially in shaded corners — you have black algae, and it's not going away without effort.
Why it's so hard to kill
Unlike green algae, black algae forms structural cells with a hard protective layer. A normal 2 ppm free chlorine residual can't penetrate it. You need to physically disrupt the surface, expose the roots, and sustain very high chlorine.
The black algae protocol
- Brush aggressivelywith a stainless steel brush (if plaster) or nylon+steel combo. Brush each spot until you see the underlying surface. Do not skip this — it's the whole game.
- Rub each spot with a chlorine tablet. Break a trichlor tab in half, wear gloves, and press it directly onto each black spot for 30 seconds. This delivers a concentrated dose into the cell structure.
- Raise FC to SLAM level (see our Green Algae guide for the ratio). Maintain for 3–5 days.
- Daily brushingwhile SLAMing, with the stainless brush. Don't stop until the spots are completely gone, not just “mostly faded.”
- Super-sanitize the filter at the end. Black algae spores live in filter media. Chem-soak cartridges overnight, backwash-rinse sand filters heavily.
When to call a pro
If black algae recurs after one SLAM, or if it's spread across more than 10% of your surface, call a pro. Severe cases may need plaster resurfacing — the algae has rooted too deep to clean. We can diagnose the extent in one visit.
Prevention
- Brush shaded corners and steps weekly — black algae loves the shade.
- Keep FC at the right ratio for your CYA.
- After any algae event, chem-soak the filter even if it looks clean.