How to Fix a Green Pool
Reviewed April 2026
Use measured CYA to choose the treatment FC target, then recalculate each addition from fresh FC. Maintain that target until all completion checks pass.
Calculate shock doseEnter verified gallons, CYA and fresh FC for this addition.
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What you'll need right now
- Cal-hypo (65% available chlorine; 68% ingredient): Check price on Amazon →
- Test kit (Taylor K-2006): Check price on Amazon →
- Pool brush (for agitating algae): Check price on Amazon →
- Pool clarifier (for clearing dead algae): Check price on Amazon →
Green or cloudy water can indicate algae; clear green water can also be metal staining. Test before adding chemicals. Rain can dilute FC and CYA; stabilized chlorine additions can raise CYA. CYA increases chlorine requirements continuously, with no chlorine-lock cliff at 80 ppm.
1. Establish valid readings
Measure volume, FC, CYA and pH. Before starting high-chlorine treatment, establish valid pH in 7.2–7.8. Recent shock can distort phenol-red tests: never add acid from an unreliable result. A treatment-range FC test is needed to measure and maintain elevated FC.
2. Choose the target from measured CYA
Use the shock calculator’s CYA/FC chart. At CYA 50, the ordinary algae target is FC 20 ppm. Unknown CYA cannot choose a target; outside CYA 20–100 this calculator declines to extrapolate beyond its sourced chart. A dilution percentage describes water exchange, not permission to drain. Before substantially lowering water, have a qualified pool professional assess construction, liner/support needs and groundwater, and choose a suitable exchange plan. Groundwater can lift a shell; unsupported liners or walls can be damaged. Account for the replacement water’s chemistry.
3. Add the measured gap
Calculate from the fresh FC reading. Liquid chlorine avoids adding CYA or calcium; if using cal-hypo, account for its calcium contribution and match available chlorine, not ingredient purity. Follow the exact product label for this formulation and your pool surface. Some granular chlorine labels require pre-dissolving; others prohibit it. Never mix chemicals, reuse contaminated containers or add water to concentrated chemicals. Keep swimmers out during treatment.
4. Maintain the target
Dose only the gap from a fresh FC reading to the CYA-based treatment target. Circulate and retest in about one hour initially; recalculate each addition to restore the target. Check more often if demand is rapid and never repeat a stored dose. Brush daily and filter continuously; clean according to pressure/condition and the filter manual. Recoat D.E. after backwashing as directed by its manufacturer.
5. Verify completion
Continue until all three criteria pass together: clear water with no visible algae, combined chlorine at or below 0.5 ppm, and overnight FC loss at or below 1 ppm (test after sunset and before sunrise, with no chlorine added or generated between tests). Then return to routine FC targets and the label’s swimming conditions.
6. Verify swimming conditions
Before swimming, measure free chlorine and pH: pH 7.2–7.8, clear water with the bottom visible, no visible algae, and FC meeting both the measured-CYA minimum and the product label’s re-entry range. CDC minimums are 1 ppm without stabilizer and 2 ppm with stabilizer; stabilized pools may need more. Test FC and pH at least twice daily and more often with heavy use. A calculated dose is not swimming clearance.
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Do not interpret cloudiness after shock as proof that algae is dead. Check for calcium precipitation and filtration problems as well. Diagnose cloudy water.
References: TFP maintenance/completion protocol; CYA/FC chart credited to Powell and Falk; CDC home-pool testing guidance. Product labels govern application and re-entry.