I'm really surprised that your ghost shrimp are doing this. I've had 4 ghost shrimp with my 6 neon tetras and 2 male guppys for weeks now in our 10g. The ghost shrimp eat off the bottom off the plants and other things in the sand. We also give them sinking food pellets. When one of our neons died we found the ghost shrimp eating it, but a lot of fish eat dead fish if you don't get them out in time. We have 3 ghost shrimp in our 55g with our cory catfish and clown loaches as well and they never bother any of those fish either. If anything the ghost shrimp are far more afraid of our fish than the other way around. I would imagine either your ghost shrimp had nothing else to eat or your fish were already sick and dying. I hope you don't flush them:( Maybe the store will take them back.