CYA – cyanuric acid, also called pool stabilizer – is the chemical that shields chlorine from UV breakdown. Without it, direct sunlight can destroy up to 90% of your free chlorine within two hours. But let CYA build up too high and you hit the opposite problem: your chlorine tests fine but can’t actually sanitize the water. The target range for most outdoor pools is 30 to 50 ppm. Get it right and your chlorine works efficiently. Get it wrong in either direction and you’ll be chasing problems all season.
What does CYA actually do in pool water?
Chlorine is unstable in sunlight. UV radiation from the sun breaks it down through a process called photolysis, and without protection, an outdoor pool can lose most of its chlorine before lunch. CYA binds loosely to chlorine molecules in the water, forming a temporary shield against UV degradation. The chlorine is still active – it still kills bacteria and algae – but it lasts hours instead of minutes when the sun is beating down. Think of CYA as a slow-release mechanism that makes your chlorine spend last much longer per dose.
The chemistry here matters: CYA doesn’t permanently tie up chlorine. It holds it in a reversible bond. When bacteria or algae show up and chlorine is needed, the chlorine breaks free and does its job, then more chlorine releases from the CYA pool. This is why stabilized chlorine (the kind in most trichlor tablets) became the default for outdoor residential pools. Each tablet slowly dissolves and releases both chlorine and CYA together.
Why do most pool owners have too much CYA?
Here’s the thing nobody tells you when you start using chlorine tablets: every trichlor tablet you drop in your floater or skimmer is adding CYA to your pool. A 3-inch tablet is roughly 54% chlorine and 46% cyanuric acid by weight. Over a full season of tablet use, CYA levels can climb from zero to well over 100 ppm without you doing anything wrong. Most pool owners don’t test for CYA regularly, so it builds up silently while they keep adding chemicals and wonder why nothing works right.
Dichlor granules do the same thing – every pound of dichlor adds about 0.9 ppm of CYA per 10,000 gallons. If you’ve been shocking with dichlor all summer on top of running tablets, your CYA can spike surprisingly fast. If your pool is losing chlorine quickly and your test strips show low free chlorine despite regular dosing, test your CYA before you do anything else. That’s usually the culprit. You can also read more about the mechanics behind this in the post on Cyanuric Acid in Pools: What It Does and Whether You Actually Need It.
What is chlorine lock, and is it real?
Chlorine lock is the informal name for what happens when CYA gets so high that it suppresses chlorine’s effectiveness to almost nothing. The chemistry is real, even if the term is informal. At CYA levels above 80-100 ppm, such a high percentage of your free chlorine is bound to CYA at any given moment that very little of it is in the active hypochlorous acid form that actually kills pathogens. Your test kit shows 3 ppm of free chlorine. Your pool still turns green. That’s chlorine lock.
The fix is not to add more chlorine. Adding more chlorine at sky-high CYA just raises your test reading without meaningfully improving sanitation. The fix is to lower the CYA first, and the only reliable way to do that is a partial drain and refill. Drain 25-50% of your pool water, refill with fresh water, and retest. Repeat if needed. There are also CYA-reducing products on the market – we make one at AquaDoc specifically for this situation – but for extreme cases (CYA over 150 ppm), a drain is usually faster and more predictable.
How to get CYA into the right range
If your CYA is too low (under 30 ppm) or you’re starting with a fresh fill, adding stabilizer is simple. Use granular cyanuric acid, not a liquid version. Dissolve it in a bucket of warm water first – this step matters because CYA granules dissolve slowly and dumping them dry into the skimmer can clog pipes or damage equipment. Add the dissolved solution slowly around the pool perimeter with the pump running. The standard dose is about 4 oz of granular CYA per 10,000 gallons to raise levels by 10 ppm. Wait 24 hours before retesting, since CYA takes time to fully register.
If your CYA is in the 50-80 ppm range and you want to keep it there, the main lever you have is switching away from trichlor tablets for routine chlorination. Use liquid chlorine (sodium hypochlorite) or calcium hypochlorite granules for your daily or weekly chlorine maintenance. Neither of those adds CYA. Reserve tablets for situations where you genuinely want slow-release stabilized chlorine, like when you’ll be away for a week. This approach, sometimes called the “pool store method in reverse,” keeps CYA from slowly creeping up year over year.
For a deeper look at when to add stabilizer versus when you already have too much, the post on Pool Stabilizer: When You Need It, When It’s Already Too High covers the decision tree in detail.
CYA targets by pool type
- Standard outdoor chlorine pool: 30-50 ppm. Below 30 and you’re burning through chlorine unnecessarily. Above 50 and you’re starting to suppress chlorine efficiency.
- Salt chlorine generator pool: 60-80 ppm. Salt pools benefit from slightly higher CYA because they’re continuously generating chlorine, and higher CYA extends how long each dose lasts between generation cycles.
- Outdoor pool in intense sun (Arizona, Florida, similar climates): Up to 50 ppm is fine; some operators push to 60 ppm in high-UV regions, but don’t go higher trying to compensate for sun – you’ll create more problems than you solve.
- Indoor pool: Zero. No CYA needed. There’s no UV to protect against, and adding stabilizer just makes chlorine less effective with no benefit.
The most common CYA mistakes
Mistake one: never testing for CYA. Basic test strips often skip CYA entirely, and pool owners assume their water is balanced because pH and chlorine look fine. Add a CYA-specific test to your routine at least once a month during the season.
Mistake two: shocking with dichlor week after week. Dichlor is a fine shock in a pinch, but it adds CYA every time you use it. For regular shock treatments, use cal-hypo or liquid chlorine instead. Save dichlor for emergencies.
Mistake three: trying to lower CYA with enzymes or algaecides. These don’t affect CYA levels. The only ways to lower CYA are dilution (partial drain and refill) or a chemical CYA reducer. Everything else is wishful thinking.
Mistake four: adding CYA and retesting an hour later. CYA takes 24-48 hours to fully dissolve and register on a test. If you retest too soon and see a low number, you may double-dose accidentally. Wait a full day before deciding whether to add more.
The independent pool service pros at River Pools and Spas say it well: most of the “mystery” problems they get called in to fix trace back to CYA being ignored for an entire season. Test it, manage it, and a lot of other problems stop happening.
Frequently Asked Questions
What should CYA level be in a pool?
For most outdoor pools, keep CYA between 30 and 50 ppm. Salt pools can run slightly higher, around 60-80 ppm, because they produce chlorine continuously. Indoor pools don’t need CYA at all since there’s no sunlight to degrade chlorine.
What happens if CYA is too high in a pool?
High CYA – above 80-100 ppm – causes chlorine lock, where your chlorine reads fine on a test but barely sanitizes anything. Algae blooms and cloudy water are common symptoms even when chlorine appears adequate. The only reliable fix is a partial drain and refill to dilute the CYA down.
Does CYA go down on its own?
No. CYA does not break down from sunlight, heat, or normal chemical use. The only ways to lower it are draining and refilling a portion of the pool, or using a CYA-removing product. This is why it accumulates over time without you noticing.
Can you swim with high CYA?
Yes, high CYA is not directly harmful to swimmers. The problem is that it makes your chlorine less effective, which means bacteria and algae can thrive even when your chlorine test looks normal. The water may look and smell fine but not actually be safe.
How do I add CYA to my pool?
Dissolve CYA granules in a bucket of warm water first, then pour slowly around the pool perimeter with the pump running. Do not dump it directly into the skimmer – it dissolves slowly and can clog or damage equipment. Run the pump for at least an hour after adding.
CYA is one of those things that rewards a little attention and punishes neglect. Once you start testing it monthly and managing which chlorine source you use, you’ll spend less money on chemicals and stop wondering why your pool keeps fighting you.