I bought a whole bunch of very small ghost shrimp to feed my pumpkinseed a few years ago. He ate a few of them and the rest escaped climbing up airlines filters and whatever else they could get hold of. I've had crawfish a couple times too. One lived in my tank peacefully with the pumpkinseed for many years until it decided to molt outside of it's little crawfish house... once. I came home to a messy shell and a fat fish. Both were very well fed ahead of time. I guess you can only distract natural instinct for so long.