I hear mixed reviews of fish food. If you are talking about pellet and flake food you typically get lost of ash in that type, while not bad it definitely doesn't do anything for your fish.
Preparing different foods for your fish can be extremely beneficial. If you look at most studies you'll find that processed foods and vitamins are not a supplement for natural ingestion of fruits, veggies and certain meats(in humans). I would imagine fish are similar.
Frozen is better than flake/pellet/processed and fresh/live is better than frozen.