The ingredients stated on the side of the carton are, fish derivatives, cereals, vegetable protein extracts, yeasts, molluscs and crustaceans.
Any fish food that has cereals or grains in the ingredients is not the best for fish. Fish never evolved to eat grains and some company's add wheat flour (or other types of flour) to make the food. They claim it is good for the fish and is used to help bind the other ingredients together. Basically it's a filler used to add weight to the product so they can make more money.
Vegetable protein extracts are usually from terrestrial plants and fish don't digest these as well or as easily as aquatic plants or algae.
And as Nick said, fish derivatives could be anything from fish flesh to bones and scales. Fish meal can be anything too.
A good fish food should have aquatic based meats as the first ingredients, and preferably no cereals, binding agents, preservatives or artificial colours. They should read something along the lines of: Fish, prawn, mussel, squid, algae, paprika (if it's a colour enhancing food), vitamins.
If the food has cereals in, make sure it is listed after the meats so it's at the end of the list. Most company's list the ingredients in the order with the largest quantity being first and smallest quantity being last so you want fish and prawn to make up the bulk of the food for predatory fishes, and algae being the main ingredient for vegetarian fishes.