Elephant-nose fish are carnivores that can be fed an assortment of live/fresh foods. They can be fed chopped red-wiggler worms (one of the smaller species of bait worms available), gammarus (scud) shrimp, wax worms, brine shrimp, fairy shrimp (looks like brine shrimp but are freshwater), chopped earthworms, chopped FW mussels, chopped FW clams, mayfly larvae, mosquito larvae, etc.
The species usually develops to learn their keepers and can be taught to feed from your fingers. Once this happens, it's easy to move them to a wider variety of foods since they'll trust that whatever is between your fingers will be edible.