High CYA in Your Pool — What It Means and How to Fix It

CYA, cyanuric acid, and stabilizer all refer to the same chemical — and it's one of the most misunderstood readings in pool care. When it's in the right range, it protects your chlorine from the sun. When it gets too high, it quietly destroys your chlorine's ability to sanitize the water. Here's the full story.

What Is CYA (Cyanuric Acid / Stabilizer)?

Cyanuric acid is a chemical added to pools to protect chlorine from UV degradation. In Southern California's intense sun, unprotected chlorine can disappear from a pool in as little as 2–4 hours. CYA acts like sunscreen for your chlorine — it slows down that breakdown dramatically.

A pool with zero CYA in SoCal would need chlorine added multiple times per day just to maintain safe levels. So CYA is necessary — but only in the right amount.

What Does "High CYA" Actually Mean?

Here's the part most pool owners don't know: CYA doesn't just protect chlorine — it also binds to it. The higher your CYA level, the more of your chlorine gets tied up and rendered inactive. Pool chemists call this the "chlorine lock" effect, though the technical term is reduced oxidation-reduction potential (ORP).

In plain English: at high CYA levels, your chlorine reading on a test strip can show 3–5 ppm (which looks perfectly fine) while the amount of chlorine actually available to kill algae and bacteria is a fraction of that. Your pool looks chemically balanced on paper, but it's actually undertreated.

This is why pools with high CYA develop chronic algae problems that don't respond to shocking. You're pouring in chlorine, the test strip says the level is good, but the pool stays green. It's one of the most frustrating situations a pool owner can face — and it's almost always a CYA problem.

What Causes High CYA?

The main culprit is the most common pool product on the market: trichlor tablets (also called 3-inch chlorine pucks). Every trichlor tablet contains roughly 50–60% CYA by weight. So every single tablet you drop in your chlorinator is adding stabilizer to your pool.

In a pool that's been running on trichlor tablets for 2–3 years without a significant water change, CYA levels of 150–300 ppm are common. We've tested pools in Riverside and Corona at over 400 ppm. At that point, chlorine is essentially useless regardless of how much you add.

Other sources of CYA include:

• Dichlor shock — also contains CYA, though less than trichlor tablets
• Stabilized algaecides — some contain CYA-based compounds
• CYA added directly — sometimes added intentionally, then forgotten about year after year

CYA does not break down in sunlight. It does not evaporate. It does not get consumed by the pool. The only things that reduce it are dilution (rain, splash-out, adding fresh water) or draining the pool. In a hot, dry Inland Empire climate where evaporation is high and pool owners are constantly topping off with fresh tap water, CYA climbs steadily every season.

The CYA-to-Chlorine Relationship

This is where it gets important. The recommended free chlorine level isn't a fixed number — it depends on your CYA level. The higher your CYA, the more free chlorine you need to maintain the same effective sanitizing power. This is called the minimum free chlorine (FC) to CYA ratio.

CYA Level (ppm)	Minimum Free Chlorine Needed	Status
30–50 ppm	2–3 ppm FC	✅ Ideal
60–80 ppm	4–5 ppm FC	⚠️ Manageable
100–150 ppm	7–10 ppm FC	🔴 Problem
150+ ppm	Near-impossible to maintain effective chlorine	🔴 Drain needed

Most pool test strips read free chlorine up to 5 ppm. If your CYA is at 150 ppm, you'd need 10+ ppm of chlorine for it to actually work — a level your test strip can't even detect, and a level that would irritate swimmers' eyes and skin.

How to Fix High CYA — The Only Real Answer

This is the part pool companies sometimes dance around: there is no chemical that effectively removes CYA from pool water. There are products marketed as "CYA reducers" that use enzymes or microorganisms, but results are inconsistent and slow. The only reliable fix is dilution.

For mildly elevated CYA (80–100 ppm), partial draining works:

• Drain 25–50% of the pool water
• Refill with fresh water
• Retest CYA
• Repeat if needed

For severely elevated CYA (150+ ppm), a full drain and refill is usually the most practical solution. In California, this must be done carefully to comply with local water conservation rules — many cities require draining to a sewer cleanout rather than letting pool water run into the street or storm drains.

How to Prevent High CYA Going Forward

Once your CYA is back in range, the key is switching to an unstabilized chlorine source — at least partially:

• Liquid chlorine (sodium hypochlorite) — contains zero CYA. The most popular choice for pools trying to control stabilizer creep. Less convenient than tablets but far better for water chemistry long-term.
• Cal-hypo shock — no CYA, but raises calcium hardness over time. Good for occasional shocking.
• Trichlor tablets in moderation — if you love the convenience of pucks, use them sparingly and test CYA monthly. Once you hit 60 ppm, stop adding tablets until levels come down naturally.

Many pool professionals in the Inland Empire have moved to a hybrid approach: liquid chlorine for daily sanitizing, occasional cal-hypo shocking, and trichlor tablets only in very small doses during peak summer when demand is highest.

The Bottom Line

If you've been battling a green or cloudy pool despite adding chlorine regularly, test your CYA before spending another dollar on shock or algaecide. Chances are your stabilizer is through the roof and your chlorine isn't doing much of anything.

The fix isn't complicated, but it does involve draining some water — which is where having a professional handle it properly makes a real difference, especially with California's water discharge regulations. We deal with high CYA regularly across Riverside, Corona, Norco, Eastvale, and Jurupa Valley.