A green South Florida pool usually means one of three things: chlorine dropped to zero, phosphates are feeding algae despite chlorine, or the filter isn't turning over water fast enough. Here's how to diagnose which one you have and fix it.
Diagnosis: test before you treat
Before dumping chemicals, test (or have a pool pro test):
- Free chlorine. Should be 2–4 ppm. Under 1 ppm with green water means the chlorine got consumed.
- pH. 7.2–7.8. Above 8.0, chlorine is only ~20% effective — pH out of range is the hidden cause of a lot of green pools.
- Cyanuric acid (CYA). 30–50 ppm is ideal. Over 80 ppm, chlorine is “locked” and won't kill algae even at high readings.
- Phosphates. Over 1,000 ppb (parts per billion) and you have algae food that chlorine can't keep up with.
Case 1: Low chlorine (most common)
A storm, a week of heavy use, or a vacation without service and chlorine runs out. Fix:
- Check and adjust pH to 7.2–7.4 first. High pH makes shock worthless.
- Shock to 10–15 ppm free chlorine using liquid chlorine (cal-hypo risks calcium spikes in South Florida water).
- Run filter 24/7 until clear.
- Brush walls and floor aggressively every 24 hours.
- Add clarifier after 48 hours to help filtration.
Timeline: 3–5 days for most green pools.
Case 2: High CYA locking chlorine
If CYA is above 80 ppm, no amount of chlorine fixes it. You have two options:
- Partial drain and refill to reduce CYA concentration — 25–50% water exchange.
- Non-chlorine shock (potassium monopersulfate) combined with aggressive filtration — slower but no draining.
Case 3: Phosphates feeding algae
South Florida municipal water is unusually phosphate-heavy. Even with ideal chlorine, algae can win if phosphates are over 1,000 ppb.
- Test phosphates quarterly (it's part of our water chemistry service).
- Treat with a lanthanum-based phosphate remover.
- Vacuum or filter out the precipitate that forms.
Case 4: Filtration failure
Sometimes chemistry is fine and the filter just isn't doing its job:
- Cartridge filter overdue for cleaning or rebuild (every 6–12 months in South Florida).
- DE filter grids torn — water bypasses rather than filtering.
- Sand filter channeling — sand needs replacement every 5–7 years.
- Pump not running long enough (minimum 8 hours/day in summer).
When to call a pro
If the pool doesn't clear in 5 days of proper treatment, something else is off. Possibilities: torn filter grids, failed salt cell, heavy metal stain masquerading as algae, or chlorine-resistant yellow algae. Get it diagnosed — cheaper than guessing with chemicals that don't work.
Frequently Asked Questions
Most green pools clear in 3–5 days with aggressive chemistry and filtration. Severe cases take 7–10 days. We rarely need to drain.
Need a pro to handle this?
Florida's Best Pools has serviced South Florida homes for 40+ years. CPO-licensed. Fully insured. 155+ five-star reviews.
