I am copying and pasting this from an other thread I replied to. Partial draining and re-filling is going to fix your high CYA for now, but it will go back up again if you keep using solid forms of chlorine.
To get a full understanding, re-read Pool School. However the key thing to understand is CYA and forms of chlorine.
Your pool needs CYA - it slows chlorine loss, but it also makes chlorine less effective. 50 where you are is fine (a touch high IMO, but still OK). You do not want it to go higher, or else you will start having problems. CYA does not drop on it's own, the only way to lower it is too drain some water and add fresh.
Now here comes the kicker - any kind of chlorine that you buy that comes in solid form - pucks, sticks, granules - either shock, tabs, clean and swim, or whatever else they call it, also contains CYA - that is just how it is. Using solid chlorine will raise your CYA.
That is why we say to use bleach (or, if you have a source and are comfortable with it, liquid chlorine, which is in essence stronger bleach)