How to Fix a Green Pool
Test before you add shock. Your CYA reading sets the chlorine target; fresh FC tells you how much to add. Keep that target until all completion checks pass, then test again before anyone swims.
Estimate your algae-treatment doseYou’ll need your pool’s gallons and fresh FC and CYA tests.
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Choose supplies once you’ve tested the water
- Cal-hypo (65% available chlorine; 68% calcium hypochlorite): Check price on Amazon →
- Taylor K-2006 test kit (check its treatment range): Check price on Amazon →
- Pool brush (match the bristles to your finish): Check price on Amazon →
- Clarifier (optional; check filter compatibility): Check price on Amazon →
If the water is green, find out why before adding shock. Algae is one possibility; clear green water can also involve metals. Chlorine doesn’t suddenly lock up at a particular CYA level. Stabilized chlorine adds CYA, and higher CYA means you need more chlorine. Water replacement can change FC and CYA, so test again after replacing it.
1. Test before treatment
Start with your actual gallons and fresh FC, CYA and pH readings. Bring pH into 7.2–7.8 before high-chlorine treatment, using a valid test and following the acid or base label for application. Recent shock can distort a phenol-red result: never dose acid from an unreliable reading. Use an FC test that can measure the elevated treatment range.
2. Find the FC target for your CYA
Enter your measured CYA in the shock calculator to find the ordinary-algae FC target. At CYA 50 ppm, that target is 20 ppm FC. You need a CYA test result first. The chart covers 20–100 ppm; for a reading outside that range, get pool-care advice before choosing a treatment target. A calculated dilution is a water-exchange estimate. Before substantially lowering the water, have a qualified pool professional assess groundwater, pool construction and the support needed by liners or walls. Groundwater can lift a shell; unsupported liners or walls can be damaged. Use an exchange plan suited to your pool and account for the replacement water’s chemistry.
3. Calculate this chlorine addition
Subtract your fresh FC reading from the treatment target. That’s the gap this dose needs to fill. Liquid chlorine adds neither CYA nor calcium; cal-hypo adds calcium. For cal-hypo, dose from the available-chlorine percentage on the label rather than the ingredient percentage. Read the chlorine label for your exact formulation and pool surface. Some granular products require pre-dissolving; others forbid it. Follow that instruction. Never mix chemicals, use contaminated containers or pour water into concentrated chemicals. Keep swimmers out during treatment.
4. Retest and restore the target
Use a fresh FC reading to calculate each addition back to the CYA-based treatment target, and apply it as the chlorine label directs. Circulate, then retest in about one hour initially. If chlorine demand is rapid, test more often. Recalculate every addition; never repeat an old dose. Brush daily and filter continuously. Clean at the filter manual’s pressure or condition trigger, and recharge D.E. after backwashing as that manual directs.
5. Check whether treatment is finished
Finish treatment only when these three checks pass together: the water is clear with no visible algae, combined chlorine is at or below 0.5 ppm, and overnight FC loss is at or below 1 ppm. Test after sunset and before sunrise without adding or generating chlorine between readings. Then resume routine FC targets and meet the label’s swimming conditions.
6. Test before reopening
A dose calculation cannot clear a pool for swimming. Before anyone swims, test free chlorine (FC) and pH. Keep pH at 7.2–7.8, with clear water, the bottom visible and no visible algae. FC must meet both the minimum for measured CYA and the chlorine label’s re-entry range. CDC minimums are 1 ppm without stabilizer and 2 ppm with it; a pool with CYA can require more. Test FC and pH at least twice daily, and more often during heavy use.
Calculate your next chlorine addition →
Still cloudy after shock? Check chlorine demand, calcium balance and the filter while you work through the treatment-completion checks. Diagnose cloudy water.
References: Trouble Free Pool algae-treatment and completion steps; CYA/FC chart credited to Powell and Falk; CDC home-pool testing guidance. Follow each product label for application and swimming after treatment.