Bottom line is: If you follow Ben's Best Guess Table for shock levels based on your CYA measurement, you'll not have to worry about CC breakdown.

The rule of thumb about vinyl liners is that with 0 CYA, you shouldn't let FC go over 15 or you could bleach or damage the liner. Note: "could". If you have CYA in your pool, you can go to the Best Guess level without fear. So I tend to think that you should always consider shocking to near 15 no matter what.

This spring I opened to an algae bloom--for the first time. Lousy weather and other commitments made my opening very late. I dumped about 4 gallons of 12.5% LC in--roughly reaching an FC of 25--and my pool was mostly clear in 24 hours and completely clear in 48. But my liner is old and ugly and STILL has footprints from the nincompoops who did the install, so I didn't care if it bleached out--it didn't and I've been fine since.