i have never heard of an emerald crab catching and eating a fish. they are one of the only known peaceful crabs , as most are a risky bet, but emeralds are not. their claws are flat for tearing off algae, and not sharp for piercing flesh. they are voracious algae eaters. not to be rude but i dont think your emeralds are the killers, just the scape goat for some other problem, because they jumped on a freshly dead meal.