CYA is something that has been added to your water as stabilizer or conditioner. It can also add up over time through the use of Chlorine Tablets, as they are stabilized. Typically, CYA does not leave your water on its own by evaporation or breakdown... high levels of CYA must be removed by replacement of water. While it is very important to have CYA in your water, having too much can start to work against you in your efforts to maintain a clear pool.