One of my favorite small predators is Stomatepia pindu.
The white spots are cephalic pits used to help locate prey that hide in the substrate.
They top off at around 8", so are a good size for a 100 gal tank.
You could keep a group. They are mouthbrooders, and are very active.
I agree about not feeding feeder fish from stores, they are disease and parasite carriers and all it takes is one disease carrier to wipe out an entire tank.
The pindu will eat safer live foods such as brine shrimp, mealworms, insects, and earth worms that might satisfy your entertainment needs.
Or you could set up a tank where you raise your own live bearers like guppies, mollies or swordtails, to provide safe feeder fish.