The Three Types of Pool Algae
Green Algae (Most Common)
- Appearance: Slimy green tint on walls and floor, cloudy green water
- Cause: Chlorine dropped below effective level, usually combined with warm water and sunlight
- Difficulty: Easy to kill with proper shocking
Yellow/Mustard Algae
- Appearance: Yellow-brown dusty patches, usually on shady walls
- Cause: Chlorine-resistant strain that survives normal chlorine levels
- Difficulty: Moderate — requires higher shock doses and brushing
Black Algae
- Appearance: Dark blue-green spots with roots that penetrate plaster
- Cause: Extremely resistant strain with a protective outer layer
- Difficulty: Hard — requires aggressive brushing, shocking, and possibly algaecide
Step 1: Identify the Problem
Before treating, confirm it's actually algae:
- Algae feels slimy and brushes off walls
- Mineral staining (copper, iron) doesn't brush off and isn't slimy
- Pollen floats on the surface and filters out easily
Step 2: Brush Everything
This is the most important and most skipped step. Brushing breaks the algae's protective biofilm and exposes it to chlorine.
- Green algae: Standard nylon brush works
- Yellow algae: Use a nylon brush and brush twice
- Black algae: Steel brush required (plaster pools only). Scrub each spot individually
Step 3: Shock to Breakpoint
Algae treatment requires reaching breakpoint chlorination — a free chlorine level high enough to overwhelm the algae completely.
| Algae Type | Target FC Level | Typical Shock Dose (10K gal) |
|---|---|---|
| Green (mild) | 10 ppm | 1 gallon liquid chlorine |
| Green (heavy) | 20 ppm | 2 gallons liquid chlorine |
| Yellow/Mustard | 20–30 ppm | 2–3 gallons liquid chlorine |
| Black | 30–40 ppm | 3–4 gallons liquid chlorine |
Critical: These doses assume a CYA level of 30–50 ppm. Higher CYA requires proportionally more chlorine. Use our pool shock calculator for exact amounts.
Step 4: Run the Filter 24/7
After shocking:
- Run the pump **continuously** for 24–48 hours
- Backwash or clean the filter every 8–12 hours during treatment
- Don't use the pool until FC drops below 5 ppm
Step 5: Brush Again
12–24 hours after shocking, brush all surfaces again. Dead algae will have loosened and the filter needs to catch it.
Step 6: Vacuum to Waste
If you have significant dead algae on the floor, vacuum to waste rather than through the filter. This prevents clogging and recontamination.
Prevention
The best algae treatment is prevention:
- Maintain proper FC relative to CYA — The FC/CYA ratio should be at least 7.5%
- Brush weekly — Even without visible algae
- Run the pump 8–12 hours daily — Stagnant water grows algae
- Check chlorine after rain — Heavy rain dilutes chlorine rapidly
- Test CYA every month — If CYA is too high, chlorine can't do its job
When to Use Algaecide
Algaecide is a supplement, not a replacement for chlorine:
- Use **copper-based algaecide** for persistent yellow or black algae
- Use **polyquat 60** as a weekly preventive (won't stain or foam)
- Never use algaecide as your primary treatment — it's too slow
Track Chemistry Trends
Algae usually appears because chlorine dropped at some point. PoolOps tracks every reading at every stop, so you can identify when and where chemistry slipped.