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Cyanuric Acid (CYA) is used to protect or stabilize the chlorine from the sun in outdoor pools. The UV light from the sun can decimate your chlorine in hours if not properly stabilized. Cyanuric acid should be kept between 20 ppm to 99 ppm to ensure proper sun protection and chlorine effectiveness.
Some chlorine tablets and granules already contain cyanuric acid, and using them will raise your cyanuric acid slowly over time.
Low Cyanuric Acid, less than 20 ppm
Low cyanuric acid will allow your chlorine to be quickly consumed by sunlight. To raise your cyanuric acid you'll need to purchase cyanuric acid which can sometimes be called chlorine conditioner or chlorine stabilizer. Carefully follow the dosage instructions to raise your cyanuric acid to 60 ppm. Be careful not to raise your cyanuric acid too high, as there is no easy way to lower cyanuric acid other than to partially drain your water.
High Cyanuric Acid, greater than 99 ppm
High cyanuric acid will prevent your chlorine from being effective. If you are using a copper based mineral purifier (such as PoolRx) and your pool has high cyanuric acid your water may also turn purple.
Cyanuric acid will not dissipate over time. The best way to lower cyanuric acid is to dilute your pool. You should drain 20% of your pool's water, refill it, and re-measure your cyanuric acid levels. then repeat the process until the cyanuric acid levels are acceptable. Under no circumstances should you entirely drain your pool because doing so may cause your pool to raise out of the ground.