What a confusing chemical we have here. The confusion begins with the fact that there are no less than three names that it goes by. Stabilizer, conditioner, and cyanuric acid (CYA) all describe the same chemical. To make it a little more confusing, chlorine tablets and some shocks contain CYA. Dichlor and trichlor chlorine both have CYA added. Calcium hypochlorite (cal-hypo) shock does not.
The purpose of CYA is to prevent the sun from burning the free chlorine out of the water. Without a stabilizer, the sun may burn out your shock in just a few hours. As well as being combined into a few types of chlorine, CYA may also be added as a granule or liquid.
Here is the trouble with CYA. If you are using chlorine tablets, your CYA level should be around 30-40. But since tablets contain CYA, it is easy to let it get out of control. If it becomes too high then you have to drain water from the pool. In a salt pool, we keep CYA higher, up to around 80. Careful though because when CYA gets too high, the chlorine becomes ineffective. If it has become too high, the pool usually will become cloudy and light green. The common reaction is to add more chlorine, thinking that this will clear it up. Your chlorine level will then be high, but the pool will not clear up. The CYA level at this point is probably 150 or higher.
I have recently read that CYA actually concentrates near the surface, so I think that if you are draining water to lower CYA, you will benefit by removing it from the top. Lastly, CYA is the mystery chemical that 90% of people don’t know about or don’t understand. It is usually responsible when you can’t figure out why the pool just won’t clear up.