Green pool water is an algae bloom. Once algae is established, standard chlorine dosing is insufficient — a shock chlorination at 30 ppm or higher is required, combined with brushing, continuous filtration, and follow-up chemistry.
Key Facts
- Green algae blooms typically start when FC drops below 1 ppm for 24 hours or more.
- The shock dose for an active green pool is 30 ppm FC — ten times the normal level.
- Brushing algae off surfaces is essential — dead algae becomes filter media that reduces filter efficiency.
- Do not add algaecide before shock — algaecide is a maintenance supplement, not a treatment for established algae.
Why Pool Water Turns Green
Algae spores are present in virtually all outdoor pool water. They are kept from blooming by adequate free chlorine. When FC drops below the effective minimum — often due to high CYA, high pH, extended sun exposure without adequate stabiliser, or missed maintenance — algae begins to multiply rapidly. A single warm day with zero free chlorine can start a visible bloom within 24 hours. Once visible, the algae has already established a biofilm on pool surfaces that must be physically disrupted to allow chemicals to reach all the cells.
The Green Pool Treatment Protocol
Step 1: Test pH, alkalinity, and CYA. Adjust pH to 7.2 (lower end — maximises chlorine activity). Step 2: Brush all surfaces vigorously to break up biofilm. Step 3: Calculate and add enough shock to raise FC to 30 ppm for a green pool (use the shock calculator). Use liquid chlorine or cal-hypo. Step 4: Run the filter 24 hours continuously. Step 5: Test FC every 8 hours. If FC drops below 10 ppm overnight, add more chlorine to maintain the high level. Step 6: Once the water turns from green to cloudy grey-white (dead algae), vacuum to waste or backwash frequently to remove dead cells. Step 7: Allow FC to return to normal range before re-entry.
After the Bloom
Once water is clear, run a full water test and correct any out-of-range parameters. Dead algae that was not removed by filtration or vacuuming may leave a dull grey haze — a clarifier dose and one more backwash cycle will address this. After any algae event, check whether CYA is above 80 ppm (a major contributing factor) and plan a partial drain if necessary. Identify the root cause of the FC drop that allowed the algae to establish — typically a missed maintenance day, equipment failure, or a storm — and adjust your routine to prevent recurrence.
Examples
A homeowner returns from a week away to find a 15,000-gallon pool turned bright green. FC is zero, pH 8.2, CYA 60 ppm. Friday evening: lower pH to 7.2 with acid. Brush all surfaces. Add 6 gallons of liquid chlorine (10% sodium hypochlorite) to reach approximately 30 ppm FC. Run filter continuously. Saturday morning: water is a murky teal-grey — algae is dying. FC reads 12 ppm — add more chlorine to maintain above 10 ppm. Saturday evening: water is hazy grey. Vacuum dead algae to waste. Sunday: water is clear. Run full test and balance chemistry.
Common Mistakes to Avoid
- Adding algaecide to a green pool without shocking first — algaecide assists healthy chemistry but cannot overcome an active bloom.
- Turning off the filter after shocking because the water looks clear — the dead algae is still in suspension and will cloud the pool again within hours without filtration.
- Not addressing the root cause (typically high CYA or missed maintenance) that allowed the algae to bloom, leading to a recurring problem every few weeks.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference