Why is my pool green (and how to fix it fast)
Updated:
A green pool is almost always an algae bloom caused by too little chlorine. The fix is a big, sustained dose of chlorine: shock to the right level and hold it there, plus good circulation and filtering. How long it takes depends on how heavy the bloom is and how often you can re-test and re-dose.
Why low chlorine turns a pool green
Free chlorine is what keeps algae in check. The MAHC Annex explains that cyanuric acid (CYA) binds chlorine and slows how fast it works, so a stabilized pool can read 2 or 3 ppm and still grow algae if that is below the minimum for its CYA. Trouble Free Pool’s chart puts the minimum at 7.5% of CYA: 3 ppm at CYA 40 and 6 ppm at CYA 80. Clearing a bloom takes much more than that, held for as long as the algae keeps using it up.
The numbers for clearing a green pool
| Step | Target | Source |
|---|---|---|
| pH before you start | 7.2–7.5 | TFP SLAM checklist |
| CYA before you start | 30–70 ppm | TFP; above 70, replace water to 60 or below first |
| Shock level | 40% of CYA (CYA 40 means 16 ppm) | TFP chart |
| Re-dosing | 3–4 times a day, at least 2 hours apart | TFP SLAM checklist |
| Filter | Clean or backwash at 25% above clean pressure | TFP SLAM checklist |
| Done when | Combined chlorine 0.5 ppm or lower, overnight loss 1 ppm or less, water clear | TFP SLAM checklist |
Step by step
- Test pH and CYA, then lower pH to 7.2–7.5Lower pH with muriatic acid before you shock, because once free chlorine passes 10 ppm the pH test stops reading correctly. Note your CYA too, since it sets the shock level. Trouble Free Pool’s checklist raises CYA to 30 if it is lower, and replaces water to bring it to 60 or below if it reads above 70.
- Shock to 40% of your CYAThat is 12 ppm at CYA 30, 16 ppm at CYA 40 and 20 ppm at CYA 50. In 10,000 gallons starting from zero, 16 ppm takes about 1.28 gallons of 12.5% liquid chlorine. Turn off any salt cell or tablet feeder while you do this so the dose stays under your control.
- Brush daily and run the pump around the clockBrush the whole pool once a day, including inside skimmers, behind ladders and around lights, where algae hides from the chlorine. Keep the pump running 24 hours a day; Trouble Free Pool notes low speed is fine and filters better. Clean or backwash the filter once its pressure is 25% above its clean reading.
- Test and top up 3–4 times a dayAlgae uses chlorine up fast, so bring free chlorine back to shock level at least 3–4 times a day. The more often you can do it the faster the pool clears, but not more often than every 2 hours.
- Stop when all three end criteria pass togetherCombined chlorine at 0.5 ppm or lower, an overnight loss of 1 ppm or less (test after sunset and again before sunrise), and clear water with no algae left on the floor or walls. Then let free chlorine fall back to your normal target.
FAQ
How long does it take to clear a green pool?
It depends on how heavy the bloom is and how often you can re-dose. Trouble Free Pool’s checklist notes that testing and topping up more often clears the pool faster, and it keeps going until all three end criteria pass at once, however long that takes.
Can I swim in a green pool?
No. The MAHC requires the bottom of a public pool to be visible whenever it is open (5.7.6.1), and Trouble Free Pool applies the same test during a shock: swim only once you can clearly see the bottom of the deep end and free chlorine is between the minimum and the shock level for your CYA.
Does algaecide help?
The MAHC Annex says most algaecides work reasonably well when used by the label and paired with frequent, thorough brushing, and that copper algaecides can stain if pH is not controlled. Chlorine at shock level does the clearing; Trouble Free Pool’s checklist does not use algaecide at all.
Sources
- Trouble Free Pool, SLAM Process Checklist (V2). pH, CYA limits, dosing frequency, brushing, filter pressure and the three end criteria
- Trouble Free Pool, CYA Chlorine Relationship (Chlorine/CYA chart). Minimum free chlorine and shock level as a share of CYA
- Trouble Free Pool, Perform the Overnight FC Loss Test (OCLT). The overnight chlorine loss test
- CDC, Model Aquatic Health Code, 5th edition (2024), Annex. Sections 5.7.3.1.3.1 (CYA slows chlorine) and 5.7.4.4.4 (algaecides)
- CDC, Model Aquatic Health Code, 5th edition (2024), Code Language. Section 5.7.6.1: the bottom must be visible
Related
Stop guessing. Get the exact fix
DoseMyPool turns your test numbers into the precise doses and the safe order to add them. Free on the App Store.