I have an apparent high CYA level 151, confirmed at pool store twice. The previous home owner left multiple pucks in skimmers and floating chlorinator. I have two friends who both "gasped" when I told them my apparent level. I ordered a TF-100 yesterday. One friend said I will have to drain off "some" of the water. Another friend who used to take care of pools said "all" and scrub the plaster walls, rinse and drain off residual. I've heard horror stories about draining pools and causing thousands of dollars worth of damage from pools floating out of the ground. Suggestions?