Eventually your lionfish will eat everything, including air bubbles, flakes and smaller fish. However, it is an ambush predator, and its brain is attuned to attacking live organisms, so you have to take advantage of that behavior. Live feeder fish are ok for a start just to get your fish eating, but plan to convert it to silversides, shrimp, scallops, Formula 1, etc. This is done by, as advised above, mimicking live food by taking the prepared food and moving it in front of the lionfish. At first it will be reluctant, but they do learn. The great thing about predatory carnivores is that they always learn what they have to do to get food.