How to Fix a Green Pool

Why Your Pool Turned Green

Green pool water means algae growth. Algae blooms when free chlorine (FC) drops low enough that the sanitizer can no longer keep up with the biological load. Usually below 1 ppm FC in an unprotected pool (the CDC Model Aquatic Health Code sets 1 ppm as the minimum FC for pools with no CYA), or below the CYA-adjusted minimum in a stabilized pool.

The most common triggers: FC drops during a heat wave (heat degrades chlorine faster), after a rainstorm (rainwater dilutes FC and raises CYA if you use stabilized chlorine), or after heavy bather load on a summer weekend.

Per the CDC pool-water health code (2023 edition), residential pools should maintain a minimum FC of 1 ppm in pools with zero CYA, scaling up to 7.5 ppm minimum for pools with CYA at 80 ppm. If your CYA is 40 ppm and you let FC drop to 1 ppm, you have inadequate sanitization by CDC standards. Algae is a predictable result.

Diagnose Before You Treat

Test your water before adding anything. You need to know:

• FC (free chlorine): likely near zero
• pH: must be 7.2–7.4 for shock to work. High pH (at 7.8+) cuts chlorine effectiveness in half.
• CYA: affects how much shock you need
• Algae type: is it green (floating, water is green) or mustard (yellow-green, clings to walls)?

If your CYA is above 80 ppm, treatment will be ineffective (per CDC MAHC CYA/chlorine relationship data). The CYA blocks chlorine from working fast enough. In severe cases (CYA above 100 ppm), you need a partial drain before shocking. Otherwise you're adding expensive chemicals that won't work.

The Shock Protocol

The protocol below is adapted from the TroubleFreePool community's algae treatment guidance, a community-validated, residential-pool-tested method widely used in the U.S. pool-care community.

Algae treatment is not a one-time dump of shock. The sustained high-FC method (splitting the dose over roughly a day) kills algae more effectively than a single large dose, because it maintains lethal FC levels as the algae metabolizes chlorine.

1. Step 1: Adjust pH to 7.2. This is the most important step and the most skipped. At pH 7.2, HOCl (the active killing form of chlorine) is about 66% of total chlorine. At pH 7.8, it drops to 33%. Shocking at high pH wastes half your chemical. Get the exact muriatic acid dose.
2. Step 2: Brush all surfaces. Use a pool brush on every surface: walls, floor, steps, behind ladders. Algae forms biofilm that protects it from chlorine. Brushing breaks the biofilm.
3. Step 3: Calculate your shock dose. For green algae, target 30 ppm FC. For mustard algae, 60 ppm. Use the calculator below. It accounts for your current FC and pool volume.
4. Step 4: Add first dose at dusk. UV light degrades chlorine rapidly. A dose added in full sun loses roughly half its strength within a couple of hours without stabilizer, so dose at dusk and start with 1/3 of the total.
5. Step 5: Run filter 24/7. Run continuously through the entire treatment. With a sand or D.E. filter, backwash daily. A clogged filter can't clear dead algae.
6. Step 6: Second dose at the half-day mark, third a day in. Add 1/3 more after 12 hours. The final 1/3 goes in 12 hours after that. This maintains lethal FC while dead algae is being filtered out.

After the Shock: Clearing the Water

After 24 to 48 hours of sustained high-FC treatment, the algae will be dead. But dead algae turns the water gray or cloudy before it clears. This is normal.

The filter clears dead algae over the next day or two. Help it:

• Brush daily to push settled algae into suspension where the filter can grab it
• Backwash sand/D.E. filters daily during clearing
• Cartridge filters need a hose-rinse every 12 hours during treatment

Once the water is clear and FC holds above 3 ppm without adding more chemical (per ANSI/APSP/ICC-11 re-entry standards), vacuum the floor to waste (not to filter). Do not vacuum dead algae through the filter. It will clog the filter.

Common Mistakes

1. Shocking at high pH. At pH 7.8+, chlorine loses 50% of its effectiveness. Always adjust pH to 7.2–7.4 before adding shock.
2. Adding all shock at once. A single large dose kills surface algae but allows protected algae to survive. Split into 3 doses over a 24-hour window.
3. Using stabilized shock (trichlor or dichlor) for algae treatment.Stabilized chlorine raises CYA. If you're doing an algae treatment, use cal-hypo (calcium hypochlorite, an unstabilized shock).
4. Turning off the filter at night. Run continuously during treatment. Turning off the filter at night lets dead algae settle back to the floor.
5. Swimming too soon. FC above 5 ppm is uncomfortable and potentially irritating to eyes and skin (per ANSI/APSP/ICC-11 re-entry standards). Wait for FC to drop to 5 ppm or below.

Frequently Asked Questions

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