Should have posted a little more info sorry. The pool is a 15,000 gal inground plaster pool that was built in 2002 by the previous owners of the house (I bought the house in 2004) The water always looked good until last year when I fought with mustard algae all summer long. Before I was only ever concerned with fc, ph and ta (I didn't know better) It wasn't until the end of last swim season that I started really lurking on the forums here and finding out what was really going on with my pool. Once I found out that my cya was through the roof (the cya test kit I have only goes to 100 and if I had to guess I would say it's somewhere around 150) and that the pucks were adding more than chlorine I stopped using them and started using only bleech, keep in mind that I have never had a ph or a ta problem and always kept my fc around 2 ppm, I couldn't tell you what the cc was becasue i never tested for it...didn't know it was important. I thought about only doing a half drain but am a little concerned that after I am done the cya won't be where I need it to be. I don't think that I need to be to concerned with the pool coming up out of the ground from water as my yard is a higher than some of the surrounding neighborhood and I live in Phoenix (Chandler to be exact) and rain or standing water is not something we get alot of here, I also don't plan on letting the pool sit empty.