Pool Care Blog
What Is CYA in a Pool? (And Why It Matters in Texas)
Chemistry · March 2026 · 6 min read
CYA. You've probably heard the term thrown around by pool techs or read it on a test kit. But unless you're deep in pool chemistry, you likely have no idea what it is or why it matters.
Here's the truth: CYA is the single most misunderstood chemical in Houston pool service. It's also the reason thousands of local pools are swimming in wasted chlorine every summer while algae thrives.
What Is CYA (Cyanuric Acid)?
CYA is a chemical sunscreen for chlorine. It protects free chlorine from being destroyed by UV rays. Without it, sunlight breaks down chlorine in minutes. With it, chlorine lasts hours.
Think of chlorine as a soldier fighting algae, and CYA as body armor. The armor keeps the soldier alive longer. But too much armor makes the soldier too heavy to fight effectively.
Ideal CYA Range for Houston Pools
30–50 ppm — Optimal. Chlorine stays effective for hours without UV destruction.
Above 80 ppm — Chlorine becomes inhibited. This is called chlorine lock. Your chlorine reads high but doesn't actually work.
Above 150 ppm — Your pool is chemically broken. No realistic amount of chlorine addition will restore algae control.
How CYA Builds Up (Especially in Houston)
Most pool owners add chlorine via stabilized chlorine tablets. These tablets contain both chlorine and cyanuric acid. Each tablet adds a tiny amount of CYA to the pool.
In northern climates, pools close in winter. The pool sits idle for 3–4 months. CYA doesn't degrade, but it's not being fed new tablets either. So it stabilizes.
Houston? We use tablets year-round. Every single day from January to December, we're adding tiny amounts of CYA. Over 12 months, that accumulation is dramatic. By September, many Houston pools hit 120–150 ppm without a single "acid wash" or drain.
Worse: CYA doesn't leave the pool on its own. It accumulates forever until you remove water.
The Chlorine Lock Problem
When CYA exceeds 80 ppm, here's what happens chemically: chlorine becomes bound to the CYA molecule. It's present in the water, but it's inert — unable to kill algae or bacteria.
Your test kit reads "8 ppm free chlorine." But because of the CYA lock, your chlorine is only half as effective as 4 ppm in a low-CYA pool. You need nearly double the chlorine to compensate, which is expensive and often impossible to maintain.
Why High Chlorine + High CYA = Green Pool
You shock the pool. Chlorine reads 10 ppm. But CYA is 120 ppm. The chlorine is locked up. It can't kill algae efficiently. You add more shock. Still green. This is the nightmare scenario for Houston pools in August.
The CYA to Chlorine Ratio
Professional pool managers use a simple rule: the minimum free chlorine level required depends on CYA:
| CYA Level | Min. Free Chlorine | Status |
|---|---|---|
| 30–50 ppm | 1–3 ppm | Optimal |
| 50–80 ppm | 2–4 ppm | Acceptable |
| 80–110 ppm | 4–6 ppm | Caution Zone |
| 110+ ppm | 6+ ppm (often fails) | Chlorine Lock |
If your CYA is 120 ppm and your free chlorine is 4 ppm, you're in a losing fight. Algae wins. To fix this, you must lower CYA, not add more chlorine.
How to Lower CYA (The Only Reliable Method)
CYA doesn't degrade in sunlight or break down with any chemical treatment. The only way to lower it is to remove water.
Partial drain and refill: Calculate how much water to remove based on your target CYA reduction, then refill with fresh water. This dilutes the CYA concentration.
Quick Rule of Thumb
To cut your CYA level in half, drain and refill 50% of your pool. To reduce it by 25%, drain 25%. Simple as that.
Example: You have 20,000 gallons at 150 ppm CYA. Target is 50 ppm. That's a 2/3 reduction. You'd need to drain 66% of the pool (roughly 13,000 gallons) and refill.
Yes, this is expensive in terms of water and refill chemicals. But it's less expensive than buying chlorine all summer for a pool that won't stay clean.
Prevention: Stop the Buildup
Once you've corrected a high-CYA problem, prevent it from recurring:
Switch from tablets to liquid chlorine.
Liquid chlorine has no stabilizer. It won't cause CYA buildup. (SWG systems are neutral — they don't add CYA.)
Lower CYA every 12–18 months.
Plan a partial drain and refill annually. This is standard in Houston for professional service plans.
Test CYA quarterly.
Catch buildup early. Once you hit 100 ppm, you're in trouble.
Reduce tablet use in peak season.
Summer algae pressure is high, but tablets are the worst way to fight it. Use liquid chlorine or a salt system instead.
The Bottom Line for Houston Pools
CYA isn't evil. It's necessary. But Houston's year-round heat, constant sun, and tablet-heavy pool culture create a perfect storm for CYA buildup. By August, many pools have CYA levels that make chlorine nearly useless.
Professional pool service monitors CYA and plans partial drains when needed. DIY pool owners often don't know CYA exists until their pool turns green despite high chlorine — and by then, it's too late to prevent the algae problem.
High CYA? We'll test it, plan a strategic drain, and restore your chlorine effectiveness.
Get a CYA Test →Need Professional Pool Help?
Admiral Pool Service has been keeping Greater Houston pools crystal clear since 2019. Licensed, insured, and PHTA certified.
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