Pool Chemistry Guide
High cyanuric acid (CYA) is one of the trickiest pool chemistry problems because it builds up silently and there is no chemical that removes it. Here's what causes it, why it matters, and how to bring it back down.
Cyanuric acid (CYA), also called stabilizer or conditioner, protects chlorine from being destroyed by UV sunlight. Without CYA, the sun can wipe out your free chlorine in a matter of hours. With the right amount of CYA, your chlorine lasts much longer.
The problem starts when CYA gets too high. At elevated levels, CYA holds onto chlorine so tightly that the chlorine becomes less effective at killing bacteria and algae. Your test kit shows you have chlorine, but it is not doing its job. This is sometimes called “chlorine lock” (though the term is debated, the effect is real).
Chlorine Pools
30-50 ppm
Saltwater Pools
60-80 ppm
Too High
100+ ppm
This is the number one cause. Trichlor tablets (the pucks you put in your skimmer or floater) and dichlor granules both contain cyanuric acid. Every time you add them, you are adding more CYA to your pool. Over a season, it accumulates because CYA does not break down or evaporate.
CYA only leaves the pool when water leaves the pool. If you never drain water, backwash, or experience splash-out and rain overflow, the CYA concentration just keeps climbing.
Adding a big dose of stabilizer when you open the pool for the season on top of the CYA already present from last year. If you did not drain any water over winter, your CYA from last season is still there.
Here is the hard truth: there is no chemical you can add to your pool to remove cyanuric acid. The only reliable way to lower CYA is to replace water.
Use a CYA test (included in most pool test kits). Know your starting number so you can calculate how much water to replace.
Drain a percentage of your pool water and replace it with fresh water. If your CYA is 100 ppm and your target is 50 ppm, you need to replace roughly half the water. If your CYA is 150 ppm and your target is 50 ppm, you need to replace about two-thirds.
Wait for the new water to circulate for a few hours, then test again. CYA tests can be tricky at high levels, so test a couple of times to confirm.
To prevent CYA from climbing again, switch from trichlor tablets to liquid chlorine (sodium hypochlorite) or plain calcium hypochlorite for regular chlorination. Add stabilizer separately and only when needed.
The simplest prevention is to stop using stabilized chlorine (trichlor/dichlor) as your primary sanitizer. Use liquid chlorine or a saltwater chlorine generator for daily chlorination. Add granular stabilizer separately at the start of the season to reach your target CYA, then do not add more unless you have replaced a significant amount of water.
Test your CYA at least once a month during swim season. If it starts creeping up, you can do a small partial drain early rather than waiting until it reaches 100+ ppm.
Track your CYA levels over time so you can spot the trend. Pool Clarity logs every test and shows you how your readings change week to week.
Pool Clarity logs your test results, shows trends over time, and tells you exactly what to add. Free to use.