You walk out to your backyard and instead of a sparkling blue pool, you're staring at something that looks like a swamp. Green pool water is one of the most common problems Fresno pool owners face — and with summer temperatures regularly hitting 105-110°F, it can happen almost overnight. Here's exactly how to fix it, step by step.
Why Your Pool Turned Green
Green water means algae. Period. Algae spores are everywhere — they blow in on the wind, wash in with rain, and hitch rides on swimsuits. The question isn't whether algae will find your pool, but whether your pool chemistry can prevent it from growing. Algae blooms when:
- Chlorine drops below 1 ppm — the most common cause
- pH is too high — above 7.8, chlorine becomes significantly less effective
- CYA (stabilizer) is too high — above 80 ppm, chlorine is essentially locked up
- Circulation is poor — not running the pump long enough for the water volume
- Filter is dirty or malfunctioning — can't remove dead algae or particles
In Fresno's extreme summer heat, chlorine gets consumed at an accelerated rate. A pool that's fine Monday morning can be green by Wednesday if the homeowner skips a day of maintenance during a 110°F heat wave.
Assess the Severity
| Severity | What It Looks Like | Recovery Time |
|---|---|---|
| Light Green / Teal | Can still see the bottom, water has green tint | 24 hours |
| Dark Green | Can't see the bottom, opaque green | 2-3 days |
| Black-Green / Swamp | No visibility, may have floating debris | 4-7 days |
The Fix: Step by Step
Step 1: Brush Everything
Before adding any chemicals, brush the walls, floor, steps, and behind ladders aggressively. Algae clings to surfaces, and brushing breaks it loose so the chemicals and filter can actually reach it. Use a stainless steel brush for concrete/gunite pools (most Fresno pools are gunite) and a nylon brush for vinyl or fiberglass.
Step 2: Test Your Water
You need to know your current numbers before adding chemicals. Focus on:
- pH: Must be between 7.0 and 7.4 for shock to work effectively. If it's higher, add muriatic acid first
- CYA (cyanuric acid): If it's above 80 ppm, your chlorine won't kill the algae no matter how much you add. You may need a partial drain and refill
Step 3: Lower the pH
If your pH is above 7.4, add muriatic acid to bring it down to 7.2. In Fresno, tap water often comes in at 7.8-8.0, so this is almost always necessary. Chlorine is about 4x more effective at pH 7.2 than at pH 8.0.
Step 4: Shock the Pool — Hard
This is not a regular maintenance shock. You need to hit the algae with overwhelming force. Use calcium hypochlorite (cal-hypo) granular shock:
- Light green: 2 lbs per 10,000 gallons
- Dark green: 3-4 lbs per 10,000 gallons
- Swamp green: 4-5 lbs per 10,000 gallons
Pre-dissolve in a bucket of water before adding. Add shock in the evening — sunlight destroys chlorine rapidly, especially under Fresno's intense UV.
Step 5: Run the Pump 24/7
After shocking, run the pump and filter continuously — 24 hours a day — until the water is clear. This is not optional. The filter is doing the heavy lifting of removing dead algae particles.
Step 6: Brush Again (and Again)
Brush the pool every 12 hours during the recovery process. Dead algae clings to surfaces and won't filter out unless you knock it loose.
Step 7: Clean the Filter Frequently
Your filter will clog rapidly as it captures dead algae. During the clearing process:
- Cartridge filters: Hose clean every 8-12 hours
- Sand filters: Backwash when pressure rises 8-10 psi above clean starting pressure
- DE filters: Backwash and add fresh DE as needed
Step 8: Retest and Re-shock if Needed
After 24 hours, test chlorine levels. If free chlorine has dropped to near zero, the algae consumed it all — shock again at the same dose. Repeat until chlorine holds at 2+ ppm overnight. The water should progress from green to cloudy blue/white (dead algae) to clear.
When to Drain Instead of Treat
If your pool is black-green swamp water AND your CYA is above 100 ppm, it may be cheaper and faster to do a partial drain (50%) and refill with fresh water. This resets your CYA levels and removes a huge amount of organic matter. Never fully drain a pool in Fresno — the expansive clay soils can pop an empty shell right out of the ground.
Preventing Future Algae Blooms
- Maintain free chlorine at 2-4 ppm year-round
- Keep CYA between 30-50 ppm to protect chlorine from UV
- Run the pump 10-12 hours daily during Fresno's summer
- Brush weekly — algae can't establish on regularly brushed surfaces
- Test water at least twice per week in summer
- Keep pH between 7.2-7.6
A green pool in Fresno is almost always fixable with aggressive shocking, continuous filtration, and patience. Most light-to-moderate algae blooms clear within 48 hours. Don't panic, don't drain, and don't waste money on miracle products — chlorine and your filter are all you need.