Fish are fed pellets of food that are appropriately sized for the size of the fish. Some are the size of grains of rice and others are a bit bigger. Just as there is a range of sizes in the kibble of dog food, there is a range of sizes for the fish food. Staff often use scoops to scoop it up and disperse it across the surface of the water. The fish waste no time schooling up and eating it all. The size and the numbers of fish being fed determine serving sizes and frequency of feeding. The fish always seem hungry if you judge this by the way they quickly gather to eat the food distributed over the water in their holding areas.
The fish food include protein and a variety of fats as well as trace minerals the fish will need to grow quickly and with a minimum of health issues. The parts of the fish that are not marketable are often "recycled" and turned into fish food. Use the link below to see how commercial fish farmers are advised to address the nutritional needs of fish.