How to Fix a Green Pool

You walked outside and your pool looks like a swamp. Don’t panic — this happens more often than you’d think in Central Texas, especially after a few days of neglect, a rainstorm, or a chemical imbalance. Here’s what to do.

Why Your Pool Turned Green

Green water means algae. Algae blooms when:

• Chlorine dropped too low — even 24 hours without adequate sanitizer can trigger a bloom
• pH drifted too high — above 7.8, chlorine loses most of its killing power
• Circulation stopped — pump failure, power outage, or timer issue
• Heavy rain — dilutes chemicals, introduces phosphates and organic matter
• Cedar pollen or oak debris — feeds algae with nutrients

The Recovery Process

Step 1: Test Your Water

Before you add anything, test. You need to know where you’re starting:

• Free Chlorine — likely near zero
• pH — needs to be 7.2 or below for shock to work effectively
• CYA (stabilizer) — determines how much chlorine you’ll need

Step 2: Lower the pH

If pH is above 7.2, add muriatic acid first. Shock chlorine is dramatically more effective at lower pH. This step alone makes the difference between a 24-hour recovery and a week-long battle.

Step 3: Shock It — Hard

This isn’t a maintenance dose. You need to reach shock level chlorine:

• CYA 30 ppm: Target FC of 12 ppm
• CYA 50 ppm: Target FC of 20 ppm
• CYA 70+ ppm: Target FC of 28+ ppm (consider a partial drain)

Use liquid chlorine (sodium hypochlorite) — not granular shock, which adds CYA.

Step 4: Run the Pump 24/7

Do not turn off your pump until the pool is clear. Circulation is critical. Brush the walls and floor to break up algae clinging to surfaces.

Step 5: Filter, Backwash, Repeat

Your filter is doing the heavy lifting now. For DE and sand filters, backwash when pressure rises 8-10 PSI. For cartridge filters, you may need to clean the cartridge multiple times.

Step 6: Maintain Shock Level

Test chlorine every few hours. The algae is consuming your chlorine as fast as you add it. Keep adding until the chlorine level holds overnight without dropping more than 1-2 ppm. This is called passing the overnight chlorine loss test (OCLT).

Timeline

• Light green: 1-2 days
• Dark green: 3-5 days
• Black/swamp: 5-7 days, possibly with a partial drain

Prevention

The best cure is prevention. Maintain 2-4 ppm free chlorine at all times, keep pH in check, and run your pump long enough for full turnover. In Austin’s summer heat, that means 10-12 hours per day minimum.

Don’t want to deal with a green pool recovery? The Pool Police catches problems before they start — that’s what weekly service is for. Call (512) 300-4136.