Highly unlikely that the bass ate all 50 grass carp.... especially if they were 12 inches when stocked. Not saying they couldn't, but in my experience grass carp are very good at avoiding predation. Over a period of years I stocked nearly 100 grass carp in a 5 acre pond and didn't see any of them for the first several years. Eventually they eliminated every last bit of vegetation in the pond. Once the weeds were gone, I saw them all the time. It's a very fine line between not enough grass carp and too many. It's easy to keep adding more because you don't see them and the weeds are still there. My advice is to wait a couple years before adding more or you could end up with way too many in the pond.