Both flakes and pellets are made with a variety of ingredients according to the specific needs of each type of fish, and usually, they contain squid, algae, fish, marine fish eggs, or plankton, as well as other components.
Therefore, if you have meat-eaters, you can find both types of food made from fish or other types of animal protein. If you have herbivores, you will also be able to find a proper type of pellet or flake as well. The same applies if you have tropical fishes, cold-water fishes, and so on. Thus, when buying flakes, always look carefully at the label, to know which type will be better for your fishes.
Moreover, when it comes to nutrition, also both types of food are prepared carefully. All the vitamins, minerals, and other nutrients that your fishes need every day to have enough energy, a strong immune system, proper coloration, healthy growth, and other key biological features of a healthy fish, are contained in either pellets or flakes. In general lines, both types of food usually can contain Vitamin C, B1, B2, B3, B5, B6, B11, K, A, E, D3, and H, as well as carotenoid pigments and antioxidants. So, from that point of view, they are very similar.