When fish are confined to a cage, they require a nutritionally complete diet. In most cases, caged fish receive no natural food. A complete diet has protein and is supplemented with a complete array of vitamins and minerals. Commercial feed mills manufacture diets for a wide array of species.
Different species have different protein requirements.
Caged fish should be fed a floating diet that will stay up in the water column. Floating feed will be trapped by a feed ring. It also allows, you to observe the fish as they are feeding. Sinking feed will only fall through the cage and not eaten by the fish wasted feed. Always remember to fit the pellet size to the fish's mouth.