Algae Bloom
An algae bloom is the rapid, visible multiplication of algae in pool water, resulting in green, cloudy, or discolored water.
Definition
An algae bloom is the rapid, visible multiplication of algae in pool water, resulting in green, cloudy, or discolored water.
Typical Values: Shock dose for active bloom: 30 ppm FC; Prevention: maintain FC above 1 ppm at all times
In Plain Language
Algae blooms begin when free chlorine drops below the effective minimum (often below 1 ppm, or below the CYA-adjusted minimum for stabilised pools) for more than 24 hours. Warm water, sunlight, and nutrients from bathers accelerate bloom development. Green algae is the most common. Black algae is more resistant and establishes as a biofilm on pool surfaces.
Why It Matters
Algae blooms make water unsafe for swimming and require 24–72 hours of intensive treatment to clear.
Typical Values
Shock dose for active bloom: 30 ppm FC; Prevention: maintain FC above 1 ppm at all times
Last reviewed: 2026-06-01