What is pool stabilizer (CYA)?

Cyanuric acid (CYA) is a pool chemical that shields free chlorine from UV degradation. Without CYA, direct sunlight destroys most free chlorine within 30–60 minutes. At 30–50 ppm CYA, chlorine lasts most of the day.

What happens if CYA is too high?

When CYA exceeds 80 ppm, chlorine becomes largely chemically inactive — a condition called chlorine lock. The pool won't clear even with repeated shocking because the chlorine can't work effectively at normal doses.

How do I lower CYA in my pool?

The only way to lower CYA is dilution — partial drain and refill with fresh water. CYA does not evaporate or break down with chemicals. To cut CYA in half, drain and refill approximately half the pool volume.

No. CYA does not evaporate. It only leaves the pool through water displacement (rain overflow, splash-out, or backwashing) or deliberate dilution by draining and refilling.

Pool Stabilizer (CYA) Guide: Too Much, Too Little, and How to Fix It

Q: How often should I test CYA?

Test CYA once a month during the swimming season, and any time you've added a significant amount of stabilized chlorine (trichlor tabs). CYA rises slowly but steadily if trichlor is your primary sanitizer.

What Is CYA / Pool Stabilizer?

Cyanuric acid (CYA) is sold as "pool stabilizer" or "pool conditioner." Its job is to protect free chlorine from ultraviolet (UV) degradation from sunlight.

Without CYA, direct summer sunlight destroys up to 90% of your free chlorine in 2 hours. You could shock a pool to 10 ppm FC in the morning and be back to 1 ppm by afternoon. With the right CYA level (30–50 ppm), chlorine lasts most of the day and only requires normal daily topping off.

CYA forms a weak temporary bond with chlorine molecules, shielding them from UV. When a chlorine molecule destroys a pathogen or algae cell, the bond breaks and CYA is recycled — ready to protect the next chlorine molecule. This is why CYA doesn't get "used up" the way chlorine does.

CYA Target Range and What Each Level Means

CYA Level	Status	Chlorine Behavior	Action
0–20 ppm	Too Low	UV destroys most FC within 1–2 hours. Can't maintain safe levels outdoors.	Add cyanuric acid to raise to 30–50 ppm
30–50 ppm	Ideal	Chlorine lasts all day. FC 2–4 ppm is achievable and sustainable.	Maintain — this is the target
50–80 ppm	Elevated	Still workable. Chlorine is somewhat inhibited — target FC 3–5 ppm to compensate.	Monitor closely; avoid adding more stabilizer
80–100 ppm	Too High	Chlorine largely inactive (chlorine lock). Pool won't clear even with repeated shocking.	Partial drain and refill to dilute
100+ ppm	Critical	Chlorine is essentially useless. You can shock all day and the pool stays green.	Major drain and refill — no chemical shortcut exists

Signs Your CYA Is Too High

High CYA is subtle — the pool looks like it should be fine (FC reads 2–4 ppm, you're adding chlorine regularly) but it just won't clear.

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Pool won't clear despite repeated shocking — you've added shock 3, 4, 5 times and the water is still green or hazy

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FC reads normal but water is green or cloudy — chlorine is present but not effective; CYA is neutralizing it

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Algae keeps coming back — a few days after clearing, the pool turns green again; chlorine can't maintain the kill

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Very high CYA test result — if your test shows 80+ ppm, that's the problem even if everything else looks fine

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Using trichlor tabs all season — each 3" tablet adds ~6 ppm CYA to the pool; a full season of tabs can push CYA to 100+ ppm

🔍 Pool won't clear? PoolDiag checks whether high CYA is the root cause based on your readings.

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How to Lower CYA: Dilution Is the Only Fix

This is the part most pool owners don't want to hear: there is no chemical that removes CYA from pool water. CYA does not evaporate, does not break down with shocking, and cannot be filtered out. The only way to lower it is to physically replace water.

How Much to Drain

To cut CYA by a specific amount, drain that fraction of the pool's total volume and refill with fresh water:

Cut CYA by 50%: drain and refill approximately half the pool
Cut CYA from 100 to 50 ppm: drain and refill ~50% of pool volume
Cut CYA from 80 to 40 ppm: drain and refill ~50% of pool volume
Cut CYA from 60 to 30 ppm: drain and refill ~50% of pool volume

After refilling, retest and re-balance all chemistry — pH, alkalinity, calcium hardness, and chlorine will all need adjustment after dilution.

How to Raise CYA

If CYA tests below 20 ppm — common at the start of the season or after a major water change — you need to add stabilizer.

Product: Granular or liquid cyanuric acid ("pool stabilizer" / "pool conditioner")
Dose: Approximately 13 oz of granular CYA raises a 10,000-gallon pool by 10 ppm
Method: Pre-dissolve in a bucket of warm water, then pour slowly near a return jet with the pump running
Timing: CYA takes 24–48 hours to fully disperse and register accurately on a test
Do not: put CYA directly in the skimmer basket — it can clog and damage the filter

Why You Can't Shock Through High CYA

This is the core of what's called "chlorine lock." To kill algae, free chlorine must be activated — and at high CYA levels, the CYA binds to chlorine so strongly that the effective (HOCl) fraction becomes extremely small.

The math: to effectively sanitize at CYA 80 ppm, you would need free chlorine of approximately 24+ ppm — nearly impossible to achieve and maintain in practice. You can shock repeatedly, FC reads 10 ppm, but the effective fraction doing the actual killing is less than 0.5 ppm.

This is why pools with high CYA look like they have enough chlorine but still go green. The chlorine is there — it's just chemically locked.

Trichlor Tabs and CYA Creep

This is how most pools end up with high CYA without the owner realizing it. Trichlor (trichloroisocyanuric acid) is the most common form of chlorine tablet — the kind that goes in an inline feeder, floater, or skimmer. Every pound of trichlor adds approximately 6 ppm of CYA to the pool.

Over a full swimming season, regular tab use can easily push CYA from 40 ppm to 80–100+ ppm — especially in pools with low splash-out and no backwash drain. The pool chemistry looks "fine" in May, drifts to unmanageable by August.

What to do instead

Once CYA is above 60 ppm, switch your daily sanitizer to liquid chlorine or cal-hypo — neither adds CYA
Use trichlor tabs sparingly — they're convenient but accumulate over time
Test CYA monthly so you catch creep early, before it requires a major drain

CYA Frequently Asked Questions

Can I add too much stabilizer?

Yes, and it's a very common mistake. Every pound of CYA added to the pool stays there permanently (until you drain). Once CYA is above 80 ppm, chlorine becomes largely ineffective and the only fix is water replacement. Add stabilizer in small doses and retest before adding more.

Does CYA evaporate or break down over time?

No. CYA is extremely stable and persistent. It only leaves the pool through water displacement — rain overflow, splash-out, backwashing, or deliberate draining. Indoor pools don't need CYA at all. If you've been adding trichlor tabs for years without draining, your CYA is almost certainly elevated.

How often should I test CYA?

Once a month during the swimming season is the minimum. If you're using trichlor tabs as your primary sanitizer, test every 2–3 weeks. CYA rises slowly, so monthly testing gives you enough notice to act before it becomes a problem.

My CYA test is hard to read — the cloudiness method is confusing. Is it accurate?

The standard "cloudiness disappears" CYA test is somewhat subjective but accurate enough for pool management. Test in consistent lighting (natural light outdoors, not indoor fluorescents). If you're unsure, test twice and average. A proper liquid reagent test kit is more reliable than test strips for CYA.

🧪 Suspect high CYA is behind your pool chemistry problems? PoolDiag diagnoses it from your readings.