It is a kind of a broad statement. Different fish has different food needs. Goldfish need food lower in protein and high in carbohydrates, bettas need a higher protein food, many tropical fish are herbovores and eat high algae foods. It is most important to learn the actual dietary needs of your fish and then feed them accordingly. And also feed them a variety of foods as well. I am sure that being fed some marine food won't hurt but a steady diet isn't necessarily better.