We have gone around this circle several times now... the pool runs fine, then at some point it won't hold chlorine levels. We have done the SLAM thing with some success, but a few times we have been told that CYA is low. We add CYA and the pool runs perfectly again.
Initially I know we had targeted the level to be on the low end (30 or 40) because I understood you don't lose CYA it usually ends up too concentrated. So the first time we added a few more pounds it seemed OK to me. It especially seemed OK when the FC immediately stabilized and all ran well. The next time we added a few more pounds had been after a long rainy summer... and again the FC stabilized right away and all seemed OK.
Now my wife came back from the pool store with a few more pounds of CYA again. When I asked her what the levels have been over time and what they are now she only knew that "CYA was OK" and now "CYA is too low"... so sorry - I'm without numbers at the moment. But am I right to be confused? Where does CYA go? If we add CYA and FC levels stabilize is that really an indicator we found the right thing to adjust?