Cyanuric Acid

Cyanuric acid is used as a chlorine stabilizer in swimming pools. It inhibits the loss of chlorine due to the sun's ultraviolet rays. Water containing cyanuric acid can maintain chlorine levels 3 to 5 times longer than water without it.

The ideal range for cyanuric acid is 30 to 50 parts per million.

Levels above 50 parts per million provide no additional advantage and much higher levels can "over stabilize" the water, which causes the chlorine to be less effective. At levels above 100 parts per million a condition known as "chlorine lock" can occur. This condition causes pools to become cloudy and increases algae growth. With chlorine lock it makes it  very difficult to maintain an adequate chlorine level.

How does the cyanuric acid level increase?

Most commercial swimming pools use stabilized chlorine tablets as their primary sanitizer source. These tablets contain cyanuric acid as a by-product and will cause the cyanuric acid levels in the water to increase over time.

How can I lower the cyanuric acid level?

The only way to decrease the cyanuric acid level is to replace the high level water with fresh tap water E.G. by draining the pool water via it's own pump system and opening the pool's fill valve to replace lost water at the same time. Replacing 50% of your pool's water with fresh water can lower the cyanuric acid level by 50%.

In most cities it is a violation of health code to keep cyanuric acid levels above 100 parts per million. This is another reason cyanuric acid levels need to be monitored frequently, to keep swimming pools safe, clean, and for economical chlorine usage.

source: A Pool Chemical With Many Names by John A. Garvin CSP