Cyanuric Acid (CYA): The Most Misunderstood Pool Chemical

Cyanuric acid (CYA), also called stabilizer or conditioner, is the most misunderstood chemical in pool care. Used correctly, it saves you money on chlorine. Used incorrectly (or ignored), it can make your pool unsafe even when your chlorine levels test "normal."

What CYA Does

CYA forms a protective bond around free chlorine molecules, shielding them from the sun's UV rays. Without CYA, sunlight can destroy up to 90% of your free chlorine in less than two hours. With CYA at 30-50 ppm, your chlorine lasts significantly longer, which means you use less of it and your pool stays sanitized more consistently.

The Sweet Spot: 30-50 ppm

For standard chlorine pools with outdoor sun exposure, 30-50 ppm is the ideal range. Below 30, chlorine burns off too fast and you're constantly adding more. Above 50, CYA starts to reduce chlorine's effectiveness. At 80-100 ppm, your chlorine is significantly weakened even though your test strip says it's at 3 ppm. At 150+ ppm, your pool is essentially unprotected regardless of chlorine level.

Saltwater pools typically run 60-80 ppm because the salt cell continuously generates chlorine, offsetting CYA's suppression effect.

How CYA Gets Too High

The most common culprit is chlorine tablets (trichlor). Every time a tablet dissolves, it adds both chlorine and CYA. Over a season, CYA accumulates because nothing in your pool removes it. Regular shocking with cal-hypo doesn't add CYA, and liquid chlorine doesn't either. But if you rely on trichlor tabs as your primary chlorine source all summer, your CYA will climb steadily.

This is why many experienced pool owners switch to liquid chlorine (sodium hypochlorite) for daily sanitizing and only use tabs when they're away from the pool for several days.

How to Lower CYA

There's only one reliable way: drain some water and refill with fresh water. No chemical on the market reliably removes CYA. If your CYA is 100 ppm and you want to get to 50, you'd need to replace roughly half your pool water. You can do this gradually, draining a few inches at a time, refilling, and retesting. Never drain more than 1/3 of your pool at once because the pressure from groundwater can damage or even pop a pool shell.

Indoor Pools Don't Need CYA

If your pool is indoors or fully enclosed, there's no UV exposure, so CYA serves no purpose. Indoor pools should have CYA at 0. If you're using trichlor tabs in an indoor pool, switch to liquid chlorine or calcium hypochlorite immediately.

The CYA-Chlorine Relationship

As CYA rises, you need proportionally more free chlorine to maintain the same sanitizing power. A general guideline: your free chlorine should be at least 7.5% of your CYA level. At CYA 40, that means FC of 3 ppm. At CYA 80, you'd need FC of 6 ppm. This is why high CYA is so problematic; it forces you into a cycle of adding more and more chlorine for the same effect.

Pool Clarity factors in your CYA level when assessing chlorine effectiveness and recommends appropriate targets based on your specific situation.