I just read in a thread that with CYA <30 not to shock a vinyl pool with more than 20 (or so) TC. When I opened, I had <30 CYA and shocked with bleach to about 24. Its down to around 5 now and crystal clear. But in hindsight my heart is in my throat that I damaged the liner (only 2 years old). And before this yuear I am embarassed to say that I have shocked it without regard to CYA levels in the past. Last year by mid season I had too much CYA, but not at the start of the season. I guess its a little like closing the barn door after the horse has left . . . If I did in fact damage it, what would the signs be?