Green algae is the most common and the easiest to treat. It causes the pool to turn teal, green, or dark green. It floats in suspension or clings lightly to walls and can be brushed off easily.
Mustard algae is yellow-green, powdery, and clings to pool walls and shaded areas. It brushes off easily but returns within hours. It is resistant to normal chlorine levels and requires an aggressive approach.
Mustard algae is notorious for returning after treatment because the spores contaminate pool toys, swimwear, brushes, and nets. Treating the pool water alone is not enough — clean or replace every item that was in the pool during the bloom.
Black algae (blue-green cyanobacteria) forms small dark spots with a lighter halo on plaster, grout, and rough pool surfaces. It has a protective outer slime layer and deep roots that penetrate the surface — making it the hardest type to kill.
| Algae Type | Shock Dose | Special Steps | Timeline |
|---|---|---|---|
| Green algae | 2–3 lbs/10k gal | Brush, filter, vacuum to waste | 24–72 hours |
| Yellow/mustard | 3–5 lbs/10k gal | Treat all equipment; use mustard algaecide | 5–7 days |
| Black algae | 3–5 lbs/10k gal | Steel brush; spot-treat with tablet; repeat for weeks | 2–4 weeks |
Log every shock treatment, brushing session, and filter cleaning during algae treatment. PoolLens tracks chlorine levels over time so you can confirm the treatment is working — or escalate if it is not.
Open PoolLens Free →Black algae is by far the hardest. It has a protective outer coating and deep roots that penetrate plaster and grout. It is resistant to normal chlorine and requires aggressive steel brushing, high chlorine concentrations, and black algae-specific algaecide over 2–4 weeks.
Mustard algae is a chlorine-resistant yellow-green algae that clings to walls, especially in shaded areas. It brushes off easily but returns quickly. It requires 3–5x the normal shock dose plus algaecide, and treatment of all equipment and swimwear that contacted the pool.
The algae itself is usually not toxic, but an algae bloom indicates the pool lacks adequate sanitation. Bacteria and pathogens thrive alongside algae in under-chlorinated water. Do not swim in algae-contaminated water.
Green algae: 24–72 hours. Yellow/mustard algae: 3–7 days. Black algae: 2–4 weeks minimum, sometimes months for severe cases. Complete removal of black algae sometimes requires replastering.
For green algae, algaecide is optional — adequate shock and filtration is sufficient. For mustard algae, a mustard-specific algaecide is required. For black algae, a quaternary ammonium or sodium bromide algaecide improves treatment success alongside heavy shocking.