Cyanuric Acid (Pool Stabilizer) Explained: The Chemical Everyone Overdoses

Cyanuric acid is the most misunderstood chemical in pool care: essential in the right amount, quietly ruinous in excess, and sold to you hidden inside products you did not know contained it. Here is the whole story in plain English.

What CYA actually does

Sunlight destroys chlorine, fast. Unprotected free chlorine in direct summer sun can lose half its strength in under an hour, which would make outdoor pools nearly impossible to keep sanitized. Cyanuric acid molecules loosely bind chlorine and shield it from UV, cutting that loss dramatically. Think of it as sunscreen for chlorine: SPF for your sanitizer.

The catch is that bound chlorine is also slower to react, at killing algae and bacteria, not just at dying to sunlight. More CYA means more protection and less killing power, simultaneously. That trade-off is the entire game.

The right level (and why more is worse)

• Outdoor, manually chlorinated: 30 to 50 ppm, the sweet spot of protection vs potency
• Salt water generator: 60 to 80 ppm, because the cell's slow continuous output spends all day in the sun (details in the salt startup guide)
• Indoor: essentially none needed

As CYA rises, the free chlorine needed to sanitize rises with it, roughly 7.5 percent of the CYA number. At CYA 40 you need about 3 ppm. At CYA 120, you need 9 ppm just to break even, which is why chronically high-CYA pools grow algae while their owners insist the chlorine "tests fine."

How pools get overdosed without anyone adding "stabilizer"

Two of the most popular chlorine products are chlorine chemically bonded to cyanuric acid:

• Dichlor (most granular shock): every 1 ppm of chlorine adds about 0.9 ppm of CYA
• Trichlor (most slow-dissolve tabs): every 1 ppm of chlorine adds about 0.6 ppm of CYA

A pool running trichlor tabs all season can add 60 to 100 ppm of CYA in one summer. The chlorine gets used up; the CYA stays. By August the tabs "stop working", they are working fine, but the accumulated CYA now demands double the chlorine the owner is adding. Liquid chlorine and cal-hypo add zero CYA, which is why the chlorine calculator lists product types separately, the choice matters beyond price.

Raising CYA on purpose (the right way)

When you genuinely need it, new fill, spring opening after dilution, use pure cyanuric acid: about 13 oz per 10 ppm per 10,000 gallons, exact dose from the chemical calculator. It dissolves slowly: put it in a sock in front of a return jet, and do not trust a test reading for up to a week. Target the bottom of your range, you can always add, and removing is a drain-and-refill job.

Lowering CYA (there is only one real answer)

Dilution. Drain a portion and refill: going from 100 ppm to 50 ppm means replacing about half the water. "CYA reducer" products exist but results are inconsistent at best. After any major refill, rebalance everything in the standard order (alkalinity, pH, CYA, chlorine) and recompute salt with the salt calculator if you have a generator. Your volume number drives the drain math, get it right with the volume calculator.

The practical takeaways

1. Test CYA monthly in season; it is the reading that reinterprets every other reading.
2. Chlorinate routinely with liquid chlorine or a salt cell; save dichlor/trichlor for when you WANT their CYA.
3. Scale your chlorine target to your CYA, not to the test strip bottle.
4. Above ~100 ppm, stop fighting and dilute, every other fix is more expensive than water.