Of course you need fish food if you keep fish in an aquarium. The question is, what kind of food is appropriate for what fish? Luckily, that isn't rocket science as fish in the same general category eat pretty much the same thing.
You can find tropical fish food, food for koi and similar fish and items such as bloodworms or shrimp. Fish food comes in flakes or pellets, dried or frozen, each with particular advantages.