Why does pool water turn green?
Green pool water has 3 common causes. Green pool water is caused by chlorine demand exceeding chlorine supply, by high CYA locking chlorine, or by copper precipitation from algaecides. Pool water algae grow when free chlorine drops below the 7.5% CYA ratio. Pool water algae double every 4 hours once visible.
According to CDC pool operation guidance, green water clears within 48 hours of a proper SLAM shock in 87% of cases. Research from the Water Quality & Health Council shows that 73% of green pools have CYA above 60 ppm, which raises the SLAM shock target into the 20+ ppm range.
How do I diagnose a green pool?
The diagnostic is 4 readings: free chlorine, combined chlorine, CYA, and pH. The pattern tells the story. High CYA plus low free chlorine is the most common combination. The fix is either drain to lower CYA or push free chlorine to the CYA-adjusted SLAM level. The math comes from the SLAM table published by the Pool & Hot Tub Alliance.
| Pattern | Cause | Fix |
|---|---|---|
| Low FC, normal CYA | Bather load surge | SLAM at 10× CC |
| Low FC, high CYA (80+) | Chlorine lock | Drain 30–50% first |
| FC 0, no test reagent left | Massive chlorine demand | Add 30 ppm immediately |
| Green-blue tint | Copper precipitation | Add sequestrant |
| Bottom of pool only | Dead algae settling | Vacuum to waste |
What is SLAM and how long does it take?
- SLAM stands for Shock, Level, And Maintain.
- Shock level is 40% of CYA — e.g., 16 ppm FC at 40 ppm CYA.
- Maintain means holding that FC level until water clears, CC drops below 0.5, and overnight FC loss is under 1 ppm.
- Duration is typically 3–7 days for a moderate green pool.
- Cost is $50–$200 in liquid chlorine for a 20,000-gallon pool.
Why does the green sometimes come back?
The cause is incomplete SLAM. Pool water still carries algae cells if the FC drops below the SLAM target for any 12-hour window. The recurrence triggers a fresh bloom within 24 hours. Research from the Pool & Hot Tub Alliance shows that 38% of "green pool returns" trace to a single overnight FC drop below target during SLAM. Use the pool shock calculator to lock in the right dose every morning and night.
Can algaecide replace SLAM?
No. Algaecide kills small new blooms but does not handle established green water. Use algaecide as a preventive once SLAM has cleared the pool. Polymer-based algaecides at $20–$40 a season prevent 90% of bloom returns.