High cyanuric acid in your pool: what it is and how to fix it
It is one of the most common and least understood pool problems. Many people try product after product without success. Here is why it happens and what really works.
What cyanuric acid is
Cyanuric acid, also called stabiliser, protects chlorine from sunlight. A moderate amount is useful. The problem starts when it builds up, because it does not evaporate and the filter does not remove it.
Where the excess comes from
It almost always comes from stabilised chlorine tablets (trichlor) and stabilised granular chlorine. Each time they are added, the chlorine gets used up but the stabiliser stays behind and keeps rising, season after season.
How to spot it
- The water turns green or cloudy even though the test shows a correct chlorine level.
- Algae keep coming back again and again.
- You shock-chlorinate and it does not improve.
At a high level, the chlorine is there but barely works. This is known as chlorine lock.
How to measure it
6- or 7-parameter test strips do not always include it. Ask a pool shop to test it for you or use a specific test kit. As a general guide, problems start above 75 to 100 mg/l.
Here is a cyanuric acid test we recommend on Amazon, or ask for one at a pool shop.
Why products won't fix it
There is no reliable product that reduces it. The only solution is to dilute the water with fresh water. And here is a very common mistake: draining only part of the pool rarely gives the expected result. You have to remove a lot of water to lower the concentration, the level stays high and, in the end, many people empty the whole pool after having already topped it up. It costs more.
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Frequently asked questions
Can you lower cyanuric acid without draining?
Only by diluting with fresh water. Products that promise to reduce it rarely give reliable results.
Is it worth draining only half?
Usually not. You have to remove a lot of water to see a change and the result tends to fall short. If the level is very high, it is better to drain and refill in one go.
How do I stop it rising again?
By avoiding continuous use of stabilised chlorine and measuring the level several times during the season.
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