How to lower cyanuric acid (CYA)
Updated:
Nothing you can pour in removes cyanuric acid. The only reliable fix is dilution: replace part of your water with fresh. The good news is the math is simple, to halve your CYA, replace half your water.
Step by step
- Confirm the reading before you drainThe standard CYA test is a turbidity test, and the MAHC Annex notes it reads no higher than 100 ppm without diluting the sample. Near or above that, mix the sample with the same amount of fill water, test, and double the result before you drain thousands of gallons.
- Work out how much water to replaceThe percentage of water you replace is the percentage CYA drops. CYA 100 down to CYA 50 means replacing half. The CYA calculator does this for your volume.
- Drain and refill in stagesNever fully drain an in-ground pool: a vinyl liner can float and a plaster shell can pop out of the ground under hydrostatic pressure. Drain partway, refill, and repeat if needed.
- Re-balance after refillingFresh fill water changes every reading, CYA included. Re-test and rebalance calcium, alkalinity, pH, and salt after the refill.
- Stop it climbing againTrichlor and dichlor add CYA with every dose; dichlor adds about 0.9 ppm of CYA for each 1 ppm of chlorine. That is almost always what pushed it up. Switch routine chlorination to liquid chlorine or cal-hypo, which add none.
FAQ
Does anything actually remove cyanuric acid?
Partial drain and refill is the dependable way. For public pools, the MAHC Annex accepts other methods only when a test afterward shows the CYA really came down, so if you try a reducer product, re-test before relying on it.
What CYA level is too high?
The MAHC caps public pools at 90 ppm and closes them at 300 ppm until it is brought down (5.7.3.1.3.2 and 5.7.3.1.3.3). Trouble Free Pool suggests 30–60 ppm for liquid chlorine pools and 60–90 for salt pools. Above that, the chlorine you need gets hard to hold: at CYA 100 its chart puts the minimum at 8 ppm and shock level at 39.
Does CYA go down on its own?
Slowly. Trouble Free Pool lists oxidation by chlorine, byproducts of sunlight breaking chlorine down, water around 85–90°F, rain, refilling after draining, splash-out and leaks as ways CYA falls. Evaporation does not remove it, so topping the pool back up does not dilute it.
Is high CYA the same as chlorine lock?
It is what people usually mean by it. The MAHC Annex explains that CYA binds chlorine reversibly and lowers the active hypochlorous acid, so chlorine tests fine and sanitizes poorly. Diluting restores its bite.
Sources
- CDC, Model Aquatic Health Code, 5th edition (2024), Code Language. Sections 5.7.3.1.3.2 (90 ppm cap) and 5.7.3.1.3.3 (300 ppm closure)
- CDC, Model Aquatic Health Code, 5th edition (2024), Annex. Section 5.7.3.1.3.1: the 100 ppm test limit, CYA binding chlorine, and reducing CYA
- Trouble Free Pool, Pool Chemical Levels: Pool Industry vs TFP (December 21, 2025). CYA targets for liquid chlorine and salt pools
- Trouble Free Pool, CYA Chlorine Relationship (Chlorine/CYA chart). Minimum and shock level at CYA 100
- Trouble Free Pool, The Free Chlorine and Cyanuric Acid Relationship. How CYA drops, and that evaporation does not remove it
Related
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