I think fish that eat primarily live foods in the wild should see a fair share of live food in our aquariums. However, the big drawback of using minnows, guppies, other common feeders ect. is that most of them are cold water fish. Cold water fish contain a lot of extra fat compared to fish in the tropics. Think of guppies, and goldfish as mcdonalds for your fish, every once and while won't hurt. But, definitely not a staple diet.
I have done a lot of reading up on shrimp/crays as feeders though. And they seem to be the best nutritionally for the fish (as well as insects). Very high in protein, low in fat, and they contain lots of fiber that is absent in most commercial pellets.
But if its fish you're interested in seeing inhaled, look into easy to breed tropical fish. I imagine convicts are much healthier.