Aquarists frequently underestimate the importance of doing this, so we continue to have problems with long term vitality, if not survival, of species kept on unnatural diets. In the case of mail-cheeked fishes, any old meat will not simply "do." Much like humans, just because a fish eats certain foods doesn't mean those foods are necessarily good for them! This sad reality plays out time and again with the Lionfish sub-family and their kin. Many, many Lionfishes meet their death from a categorically inappropriate diet of “feeder” freshwater goldfish or the like, which are expensive, hard to digest, cause for “fatty degeneration” of internal organs, are inconvenient… and, in a word, are “bad” for them. Don’t fall into the trap that your predators “need” feeder goldfish (where would they get these in the wild?) or cannot be trained to take killed prey and prepared foods. Pterois lionfishes and many of their relatives can be trained to even take dried carnivore food sticks or pellets! Make no mistake that neither these nor any such predator specializes in wasting energy to catch the fastest and hardest to capture prey. Please! They seek the young, slow, weak, dying, or otherwise unaware specimens. In captivity, however, most will adapt to killed or prepared foods (mostly thawed frozen meats of marine origin).
Above all, freshwater prey items are nutritionally deficient and will lead to the death of your marine predator in time. Merchants and aquarists alike that have let their predatory fishes "train" them into thinking freshwater feeder fishes were good, necessary, or all that would be accepted need not wonder anymore why, after a year or two, the predators begin to suffer strange ailments. These ailments include symptoms like renal/vision failure ("they sense the food in the water, but seem like they cannot see it clearly…snapping short and missing?") , or they appear to be unable to swallow the old familiar prey as if they have a "lump" in their throat (goiter, swollen thyroid). Chalk it up to dietary deficiencies or inadequacies (like thiamin deficiency induced by a staple of feeder goldfish) taking its toll over time. You should be determined to offer a wide variety of thawed fresh-frozen meats, dry, or otherwise prepared foods when possible and gut-loaded live prey if absolutely necessary. Identify if your species favors arthropods (shrimp, crabs, and the like) or fishes, and adjust the given diet accordingly.
