Cyanuric acid is a stabilizer added to chlorine products to keep it shelf stable and to help keep it stable once it is in your pool or spa.
Cyanuric acid helps to keep chlorine from being broken down prematurely when exposed to sunlight. The bad news is that it can make the free chlorine in your pool or spa less available to sanitize. However, like many things in life, balance is everything. Keeping the amount of cyanuric acid below 70 mg/L not only allows for the free chlorine to sanitize, it also keeps the concentration of cyanuric acid below toxic levels. High cyanuric acid can also cause eye and ear infections.
Too much cyanuric acid in your pool or spa? Unfortunately, the only way to lower the amount of cyanuric acid is to thoroughly clean you pool and to change out the water.
Using a system that doses liquid chlorine will keep you from having cyanuric acid accumulate as liquid chlorine does not contain cyanuric acid. It is recommended that chlorine granules and tablets not be used in public swimming areas to avoid an accumulation of cyanuric acid.