What Is Cyanuric Acid and Why Does My Pool Need It?

Cyanuric acid, usually sold as stabilizer or conditioner, is chlorine’s sunscreen. It bonds loosely with free chlorine and protects it from ultraviolet light, which would otherwise destroy most of your chlorine within a few hours of direct sun.

Every outdoor chlorine pool needs some. The target for a traditional chlorine pool is 30 to 50 ppm. Saltwater pools generally run a little higher, 60 to 80 ppm, because the generator produces chlorine continuously and needs the extra protection.

What happens without it

Unstabilized chlorine in bright sunlight has a half-life measured in tens of minutes. A pool at zero CYA can lose the majority of its free chlorine in an afternoon. You end up dosing constantly, spending far more on chlorine, and still getting algae because the sanitizer keeps vanishing between doses.

With CYA in the 30 to 50 ppm range, that same chlorine can last through the day and into the next.

The catch: too much is a real problem

CYA does not evaporate and it does not break down meaningfully. It only leaves through water replacement, splash-out or backwashing. So it accumulates, and it accumulates fastest in pools sanitized with trichlor tablets or dichlor shock, since both add CYA every time you use them.

For rough planning, every 1 ppm of chlorine added as trichlor also adds about 0.6 ppm of CYA. That adds up fast over a season.

When CYA climbs too high, chlorine gets over-protected and sluggish. This is often called chlorine lock, though the chlorine is not truly locked, it is just heavily slowed down. Symptoms look like this:

• Test shows adequate free chlorine but algae grows anyway
• Water stays dull or cloudy
• Shocking barely moves the needle
• Rising chlorine demand with no obvious cause

Above roughly 100 ppm most testing kits stop reading accurately, and health departments in many jurisdictions cap commercial pools at 100 ppm or below.

The chlorine to CYA relationship

A widely used field rule is to hold free chlorine at about 7.5 percent of your CYA level as a minimum:

• CYA 30 ppm, keep free chlorine at 2 to 3 ppm
• CYA 50 ppm, keep free chlorine at 4 ppm or so
• CYA 80 ppm, you need roughly 6 ppm to get the same effect
• CYA 100 ppm and up, the chlorine demand becomes impractical

That is the whole argument against letting CYA drift. High stabilizer forces you to run high chlorine just to stand still.

How to raise it and how to lower it

To raise CYA, add granular stabilizer. About 13 ounces per 10,000 gallons raises CYA roughly 10 ppm. Most granular products dissolve slowly, so add them through the skimmer with the pump running (or place them in a sock in front of a return) and do not backwash for two to three days.

To lower CYA, there is essentially one reliable method: dilution. Drain a portion of the water and refill. To cut CYA in half you replace about half the water. There are enzymatic CYA reducers on the market but results are inconsistent and slow.

Why this matters so much in the Keys

Key West gets intense UV nearly every month of the year. There is no low-sun winter here where a pool coasts along on minimal chlorine. That makes stabilizer non-negotiable on this island, and it also means a lot of local pools get run on tablets year-round, which is exactly the habit that pushes CYA past 100 ppm.

Because we never close a pool for a season, there is no natural drain-and-refill event to reset the number. Our high water table also makes fully draining a pool risky, since an empty shell can float or shift, so partial drains done carefully are the right approach here rather than emptying the pool.

Test cyanuric acid at least twice a year, because it is the number that quietly rises for months and then explains every other problem you have been fighting.