Green, Yellow, or Black? The 3 Pool Algae Types and How to Treat Them

Not all pool algae is the same. Green, yellow (mustard), and black algae each require different treatment — and misdiagnosing them wastes money and time. Here's how to tell what you're dealing with.

Green algae

The common one. Water turns green, cloudy, and slimy. Grows fast, dies fast.

• Cause: Low chlorine, high CYA, or high phosphates — often all three.
• Treatment: Balance pH to 7.2, shock to 10–15 ppm free chlorine, brush, filter 24/7, vacuum the settlings.
• Timeline: 3–5 days.
• Cost: $350–$800 professionally, $50–$150 DIY.

Yellow (mustard) algae

Looks like pollen or sand dusting the walls and floor — usually on the shady side of the pool. Chlorine-resistant.

• Cause: Airborne spores settling in shaded, low-circulation areas. Chlorine-tolerant.
• Treatment: Elevated chlorine (double the normal shock dose), a yellow-algaecide (ammonium-based), aggressive brushing, and cleaning everything that's touched the water — nets, vacuums, pool toys — with chlorine solution. Otherwise it re-introduces itself.
• Timeline: 5–10 days.
• Cost: $500–$1,000 professionally.

Black algae

The worst. Dark green to black spots, often the size of a coin, that embed roots into pool plaster and cement. Normal chlorine can't kill the roots.

• Cause: Rough plaster surfaces + high phosphates + poor circulation. Often originates from lake or spring water that touched the pool.
• Treatment: Mechanical removal — stainless steel brush to break through the protective cap, then high chlorine and black-algae-specific treatment. Sometimes acid washing is the only reliable fix.
• Timeline: 2–4 weeks; recurrence is common.
• Cost: $800–$2,500 professionally; acid wash $500–$1,200 if needed.

Pink slime (bonus)

Pink or red streaks at the waterline or in plumbing. Not actually algae — it's a bacteria. Usually harmless but ugly and indicates chronically low chlorine or dead spots in circulation.

• Treatment: Shock + algaecide + clean waterline tile.

How to prevent recurrence

• Weekly service with proper chemistry balance
• Quarterly phosphate treatment
• Brushing at every visit (weakens spore anchor points)
• Adequate pump runtime — 8–12 hours/day minimum in summer
• Cleaning pool toys, floats, and cleaning equipment regularly

Algae that won't clear on its own? We handle green-to-clean recovery across South Florida.

Frequently Asked Questions