Algae spores are constantly introduced into swimming pools via airborne wind currents, rainstorms, and swimwear. When free chlorine falls below the minimum required ratio relative to Cyanuric Acid, spores germinate. Treating an outbreak requires identifying the specific algae strain.
The Three Common Strains of Pool Algae
1. Green Algae (Chlorophyta)
The most prevalent strain. Floats suspended in water making it cloudy or emerald green, with slippery walls. Highly vulnerable to standard liquid chlorine shock.
2. Mustard / Yellow Algae (Phaeophyta)
Resistant to standard chlorine levels. Clings to shaded walls and pool fittings in yellow-brown powdery patches. Requires elevated shock levels (approx. 60% of CYA level) and equipment sanitization (washing pool poles, floats, and swimsuits).
3. Black Algae (Cyanobacteria)
Forms pinpoint black or dark green dots with deep root structures embedded in plaster. Resistant to standard filtration. Requires aggressive wire brushing with stainless steel bristles followed by rubbing trichlor pucks directly into the spots.
Frequently Asked Questions
How do you tell the difference between mustard algae and pollen?
Pollen floats primarily on the water surface and collects in the skimmer, easily blowing away when brushed. Mustard (yellow) algae clings to shaded walls and stairs, feels powdery, and returns in the exact same spot within 24 hours of brushing if sanitizer is low.
Why is black algae so difficult to kill?
Black algae is technically a cyanobacteria. It roots into the microscopic pores of pool plaster and forms a dense, waxy protective outer layer that repels free chlorine. It must be scraped with a stainless steel wire brush before applying direct trichlor granular contact.