Usually If you're fish is really hungry it will have nothing else but its companion(s) in the tank. My fish ate my guppy's fins and he died soon after
You have four different types of fish. Herbivores eat only plant matter, carnivores eat meat matter, omnivores eat both meat and plant matter, and scavengers eat anything that is left from every other fishes scraps. Just like creatures on land, some eat plant and seed matter like cows, horses and birds. Some eat meat like wolves, bears and predatory birds like hawks and eagles.
Because if one fish sees a fish that is smaller than him he will will try to eat it that is what you call the food chane
For living some protiens r not achieved thru normal food so they try live feed but generally I preffer chicken heart not liver but cleaning is necessary without fat and some prawns