I'm not familiar with Sustain Summer Shield and couldn't find very much information as to what's really in it, besides the label which says Calcium Hypochlorite 68% and "Other ingredients" 32%. Sounds likes it's some sort of CYA replacement? I have any had to shock my pool once, and that's when I started to get an algae bloom due to my FC levels dropping too low. I never have any CC since switching to bleach. Maybe someone else that has experience with this product will chime in with their experience.