Cyanuric acid ends up in your pool water via regular chlorine tablets and chlorine granules. This substance is added to these chlorine products to stabilise them. This stabilisation prevents the free chlorine from being decomposed by UV light.
At a high UV index, the sunlight decomposes 50% of the free chlorine per 45 min. Stabilisation is therefore absolutely required when using regular chlorine tablets or granules. These chlorine products are in fact less effective than inorganic chlorine products.
As long as the cyanuric acid concentration is less than 20 mg/l, there will not be any problem. Once a level of 70 mg/l is exceeded, cyanuric acid becomes problematic. Too high a concentration can, however, easily be solved. All you need to do is add fresh water to the pool (3 to 5% of the total volume) and lower the concentration cyanuric acid.