This is something I copied from an Aquarium Fish Magazine article:
"Of course, the best foods are those that are as fresh as possible, such as raw shrimp, clams, mussels and marine fish. For large predators, including groupers and lionfish, it is extremely important to avoid feeding them freshwater fish as a staple food. Try and wean them from these foods and onto prepared marine foods or fresh raw foods. The reason for this is that freshwater fish lack certain essential fatty acids that are mandatory for marine fish. A lionfish may live for years on a diet of goldfish then one day it will inexplicably die. When these fish are examined, they are usually found to have fatty livers caused by the abundance of improper fatty acids in their diet."
It gives a very good explanation as to why you shouldnt use freshwater fish as food.