"CYA," or cyanuric acid, is a stabilizer or conditioner that is used to reduce the rate of composition of free available chlorine in pools that are exposed to the sun's ultraviolet rays. Dissolving CYA into a swimming pool helps to make free chlorine last significantly longer.
Stabilized chlorine contains CYA, so adding more is not necessary. Outdoor pools and spas that use non-stabilized chlorine require the owner to add CYA manually. It is important to monitor CYA levels, as too little stabilizer causes the free chlorine to lack protection. Too much CYA causes the chlorine to lose its sanitizing and oxidizing properties.