My pool equipment manufacturer says that I should add 1.5 lbs of CYA every time I add a bag of salt to my pool. I'm wondering why they would suggest this. Is it because by the time that I need to add a bag of salt, the CYA is probably a little low? Or is there some other reason why they'd suggest that?

Ironically or not, in the interval between the time I need to add another 40lbs bag of salt into my pool, the CYA is down about 20 ppm or so. I try to keep CYA close to 80, so my FC level can stay around 4.0.

Thoughts?