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Why your pool won’t hold chlorine

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If chlorine disappears, you usually have one of three problems: no stabilizer (CYA), so sunlight burns it off; an algae or organic demand eating it; or high combined chlorine. Stabilize and shock as needed.

What uses up chlorine

The CDC lists what lowers chlorine: urine, feces, sweat and dirt from swimmers, sunlight, and hot tub jets that throw water into mist. In a pool that loses chlorine fast, the usual suspects are sunlight with too little CYA, or algae and other organic matter using it up. An overnight test tells the two apart, because at night the sun is out of the picture.

Step by step

  1. Check CYA (stabilizer)The MAHC Annex describes CYA as protecting chlorine from sunlight; with little or none, the sun burns chlorine off during the day. Trouble Free Pool suggests 30–60 ppm for a liquid chlorine pool and 60–90 ppm for a salt pool. The MAHC caps public pools at 90 ppm.
  2. Run the overnight chlorine loss testTurn off any salt cell or tablet feeder and run the pump for 30 minutes before each test. After sunset, test free chlorine and bring it to your target. Test again before sunrise. A loss of 1 ppm or less means the water is clean and sunlight was the problem. Trouble Free Pool notes this needs an FAS-DPD kit; strips are not precise enough.
  3. Check combined chlorineCombined chlorine is total chlorine minus free chlorine. Trouble Free Pool says to look closer above 0.5 ppm, the level at which the app calls for a shock. The MAHC sets 0.4 ppm as the level at which public pools must act, for example by superchlorinating (5.7.4.4.2).
  4. Shock until it holdsIf the overnight test loses more than 1 ppm or combined chlorine is high, shock to 40% of your CYA and hold it there until combined chlorine is 0.5 ppm or lower, the overnight loss is 1 ppm or less, and the water is clear.

FAQ

How much CYA should I have?

Trouble Free Pool suggests 30–60 ppm for a pool dosed with liquid chlorine and 60–90 ppm for a salt pool, while Hayward’s and Pentair’s salt cell manuals list 30–50 ppm. Stay at or below 90 ppm, the MAHC cap for public pools, and use none in a hot tub, as the CDC recommends. The CYA calculator gives the exact amount.

Why does chlorine drop even with the right CYA?

Then something in the water is using it: algae, or organic material from swimmers. The overnight test will show it. A loss of more than 1 ppm between sunset and sunrise means shock.

Sources

  1. CDC, Home Pool and Hot Tub Water Treatment and Testing (May 10, 2024). What lowers chlorine, and no CYA in hot tubs
  2. CDC, Model Aquatic Health Code, 5th edition (2024), Code Language. Sections 5.7.3.1.3.2 (90 ppm CYA cap) and 5.7.4.4.2 (combined chlorine action level)
  3. CDC, Model Aquatic Health Code, 5th edition (2024), Annex. Section 5.7.3.1.3.1: CYA protects chlorine from sunlight
  4. Trouble Free Pool, Perform the Overnight FC Loss Test (OCLT). How to run the overnight chlorine loss test
  5. Trouble Free Pool, SLAM Process Checklist (V2). Shock level and end criteria
  6. Trouble Free Pool, Pool Chemical Levels: Pool Industry vs TFP (December 21, 2025). CYA targets for liquid chlorine and salt pools
  7. Trouble Free Pool, Pool Chemistry ABC’s. Combined chlorine above 0.5 ppm calls for a closer look
  8. Hayward, AquaRite 900 Series Electronic Chlorine Generator manual (092623 Rev D). Stabilizer 30–50 ppm
  9. Pentair, IntelliChlor IC15/IC20/IC40/IC60 Installation and User’s Guide (P/N 520589 Rev. R, 2023). Cyanuric acid 30–50 ppm

Related

CYA calculatorShock calculatorPool chemistry 101

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