Algae Treatment & Prevention

Identify your algae

• Green algae (most common): Water turns green and cloudy, walls feel slimy. Easiest to kill.
• Yellow / mustard algae: Dusty yellow patches in shady spots. Resistant — needs higher shock levels and thorough brushing.
• Black algae: Dark spots with roots in plaster, often in cracks. Hardest to kill — requires spot-brushing with a steel brush and sustained high chlorine.

The kill sequence (in this order)

1. Brush everything. Walls, floor, steps, ladders, behind light niches. Brushing breaks algae's protective layer — chemicals alone won't cut it.
2. Balance pH to 7.2–7.4. Chlorine works far better at the low end of the range.
3. Shock hard in the evening. Green: bring free chlorine to ~10x combined chlorine. Yellow/black: go higher and hold it. Use our Chlorine Calculator for the dose.
4. Run the filter 24/7. Backwash or clean as pressure rises. No swimming until free chlorine is under 4 ppm and water is clear.
5. Vacuum dead algae, brush again, and re-shock if any green remains. Repeat until you hold 1–3 ppm free chlorine overnight with no loss.

Keep it from coming back

• Never let free chlorine sit at zero — maintain 1–3 ppm consistently.
• Run the pump enough to turn over all water daily (usually 8–12 hours).
• Keep CYA at 30–50 ppm so sunlight doesn't burn off chlorine.
• Brush weekly, even when the water looks perfect. Algae starts invisible.
• Shock weekly during heavy use, after storms, and after parties.

What not to do

• Don't just add algaecide to a green pool. Algaecide prevents algae; it doesn't reliably kill a bloom. Shock and brush first.
• Don't drain a vinyl-liner pool to fix algae. Liners can shift and wrinkle. Treat chemically.
• Don't swim in an active bloom. Visibility and sanitation are both compromised.