Cyanuric Acid for Pools: A Beginners Guide to CYA



Are you trying to understand the role cyanuric acid plays in your pool?

In this guide, we’ll take you through what cyanuric acid is, why your chlorine relies on it, how it works in the water, what causes it to change, how to properly test it, and much more.

Quick answer: Cyanuric acid is a chlorine stabilizer. It shields your chlorine from the sun’s UV, preventing it from breaking down too quickly. It’s a necessary addition for any outdoor or indoor pool that receives direct sunlight.

What is Cyanuric Acid?

Cyanuric acid, or CYA, is a weak acid that helps to protect the free chlorine in your pool from sunlight, so it’s a kind of like “sunblock for your sanitizer”.

While this powdery substance can be added to your water separately, many types of chlorine already contain a high amount of cyanuric acid, including most chlorine tablets or pucks.

If you’ve ever heard terms like pool stabilizer, pool conditioner, or chlorine stabilizer, know that it’s all referring to cyanuric acid.

Why is Cyanuric Acid Important?

Ultraviolet (UV) light degrades chlorine very quickly, breaking free chlorine down into smaller components that have no sanitizing power.

Because of this, swimming pools with lots of sun exposure need cyanuric acid to protect (or stabilize) their chlorine. This includes most outdoor pools and even some indoor pools if they get enough direct sunlight.

Without that protection, your pool can lose up to 90% of its free chlorine level in a matter of hours, leaving your water under-sanitized and extremely vulnerable to all sorts of nasty contamination.

Note: This is one of the major causes of chlorine demand, where your water requires more free chlorine than is available.

How Does Cyanuric Acid Work?

Cyanuric acid seeks out and binds with free chlorine in the water, becoming stabilized chlorine. The more cyanuric acid you have in the water, the more of your chlorine gets caught up in this process.

Stabilized chlorine is different in that it’s far more resistant to UV light, meaning it stays in the water and continues to work whereas free chlorine would usually just break down and burn off.

However, this form of chlorine isn’t very useful as a sanitizer because stabilized chlorine is far weaker than unstabilized chlorine.

To solve this, when your free chlorine runs low, cyanuric acid slowly releases chlorine from this bond so it can sanitize the water effectively. It essentially acts as a chlorine reserve, making it available only as needed.

What is the Ideal Cyanuric Acid Level?

The ideal stabilizer level is where enough of your chlorine has bonded with cyanuric acid to prevent it from breaking down, but not so much that it leaves your water without enough effective sanitizer.

For chlorine pools, this range is 30 to 50 ppm (parts per million) depending on the level of sun exposure it receives.

However, this assumes you maintain a free chlorine level of 1-3 ppm. Since CYA scales with chlorine, you must increase your cyanuric acid level when maintaining higher levels of free chlorine.

For saltwater pools, the range is a little higher at 60 to 80 ppm. This is because a salt pool maintains a much lower level of chlorine due to how chlorine is continuously produced and fed into the water.

Note: Your free chlorine should account for no less than 7.5% of your CYA level in a traditional chlorine pool, and no less than 5% in a salt pool.

Can You Use Too Much Stabilizer?

Yes, too much cyanuric acid will over-stabilize your chlorine.

When too much of your free chlorine is caught up in reserve—meaning it’s bound to cyanuric acid, it doesn’t leave enough free chlorine to effectively fight off contaminants in the water.

While it’s true the chlorine bound to cyanuric acid does eventually become regular free chlorine again, the speed at which this happens is significantly slowed down when your stabilizer level is high.

This over-stabilization can lead to an issue known as chlorine lock, where chlorine stops working despite being well within the ideal range. In reality, it’s just being suppressed by a high cyanuric acid level.

Does Cyanuric Acid Influence Other Levels?

No, chlorine stabilizer only directly impacts chlorine.

Even then, your actual free chlorine level will remain unchanged despite interacting with cyanuric acid because a testing kit will still account for stabilized chlorine as part of your free chlorine reading.

However, since chlorine directly and significantly impacts your water, it’s fair to say it still has a strong indirect influence over your pool.

What Causes Cyanuric Acid to Change?

As far as pool chemistry levels go, cyanuric acid doesn’t tend to fluctuate much and is considerably easier to manage.

Still, it’s not entirely immune to change.

What Raises Your CYA?

As silly as it sounds, the biggest cause of rising cyanuric acid in a pool is simply adding more cyanuric acid to the water.

Many pool owners continuously introduce stabilizer to their water without even realizing it. This is because most granular and tablet forms of chlorine are trichlor or dichlor, containing more cyanuric acid than chlorine.

Similarly, some pool owners may be unknowingly adding cyanuric acid to their pool through products that aren’t clearly labeled as such, including those marketed as “pool stabilizer” or “pool conditioner”.

The only other possible cause is evaporation. Cyanuric acid doesn’t evaporate with water, so it gets left behind and becomes more concentrated over time. This can be offset by topping up with unstabilized fill water.

What Lowers Your CYA?

It’s notoriously difficult to lower cyanuric acid in a pool, so this is rarely an issue you have to be concerned about.

CYA does degrade over time because bacteria and even chlorine (through oxidation) break it down over time. However, this happens at an extremely slow rate and results in an average loss of around 3% per month.

Again, the only other factor involved is the water itself. When you lose water through splash-out or backwashing, topping it up with unstabilized water will dilute and lower your CYA level. The same applies to rainwater.

How Do You Test Cyanuric Acid?

Most pool testing kits allow you to test your cyanuric acid level, along with pH, alkalinity, calcium hardness, and free chlorine.

Pool test strips are among the most common. They can be dunked directly into your water before comparing the colored strips to the provided chart. One of those colored strips will represent your CYA level.

Liquid drop kits require a sample from your pool before adding a reagent to color the water. While this is slower and more involved, it’s considerably more accurate than strips for testing any parameter, including CYA.

However, cyanuric acid can also be measured using “the black dot test” which is even more accurate than using a regular drop test. Some kits will include this test as standard, but you may need to buy it separately.

How Do You Adjust Cyanuric Acid?

If your cyanuric acid level isn’t at 30-50 ppm for a traditional chlorine pool and 60-80 ppm for a saltwater pool, you’ll need to adjust it.

Here’s a brief overview of how to do it:

How to Raise Your CYA Level

If your CYA is too low, you can raise it by simply adding more stabilized chlorine (most chlorine tablets are stabilized) or straight cyanuric acid (these are bottles labeled as pool stabilizer or conditioner).

Chlorine tablets can be placed in a feeder or floater and left to dissolve, while granular chlorine is usually pre-dissolved in a bucket of water.

Straight cyanuric acid should also be dissolved in a bucket before being added to your pool. Alternatively, you can also slow-dissolve with the “sock method” by filling a sock with CYA granules and hanging it in front of a return jet.

You can find liquid cyanuric acid at a slightly higher price-point than the granular version, but this might be worth the added convenience of being able to just pour it directly into your pool.

How to Lower Your CYA Level

Unfortunately, if your CYA is too high, lowering it isn’t as easy.

As mentioned earlier, cyanuric acid is very stubborn in the sense that it degrades very slowly and doesn’t evaporate with water. As such, waiting it out just isn’t a viable solution for reducing your stabilizer level.

While cyanuric acid reducers such as Bio-Active or Natural Chemistry claim to lower your CYA without draining, these products have been met with mixed reviews and many have expressed disappointment.

Overall, the only reliable way to lower your cyanuric acid level is to drain some of your water and replace it with fresh, unstabilized water — which can be both expensive and time-consuming.

How Do You Maintain Cyanuric Acid?

You won’t see much movement in your CYA level unless you’re frequently adding more cyanuric acid or draining and refilling your pool.

With that being said, take some time to understand exactly what you’re putting in your water, whether it be chlorine that contains cyanuric acid or pool products that promise ambiguous benefits.

If your stabilizer level is where you want it to be, consider switching to unstabilized chlorine such as sodium hypochlorite (liquid chlorine) if you haven’t already done so.

Finally, limiting water loss and dilution will help to keep your cyanuric acid (and your other levels) more stable. This includes preventing splash-out and excessive backwashing, as well as covering the pool during heavy rainfall.

The Bottom Line

If you have an outdoor pool, there’s no getting around it: this acid is necessary to protect your pool from chlorine loss.

Without it, any chlorine you add will be gone in a matter of hours, completely wasting your time and money (not to mention leaving your pool open to bacteria and algae attacks).

It’s one more chemical you’ll need to add to your pool, but with proper care and testing, will protect your chlorine, your wallet, and your sanity.

Categories: Pool Care, Pool Chemistry