"Fish is brain food," is what I would hear in college from a roommate of mine who was seriously into his health. He believed this so much that he would eat a piece of fatty fish before exams, and would urge me to do the same. I was interested in health at the time, too, so I tried it. Maybe I just didn't do it right or for long enough because I found that my friend's pre-exam ritual was a bad one, as was his reasoning for doing so, which was a physiological impossibility.
There is an ever-present belief that if a food contains a substance that is found in a particular body part, it follows inevitably that said food is beneficial for said body part. I think this is what people really mean when they say that fish is brain food. The meat of fish and the brain both contain phosphorus, and before the hype about omega-3 fatty acids, phosphorus was thought to be the reason why fish was good for the brain. Fish is actually a modest source of phosphorus, as there are many other foods that contain significantly more, and phosphorus is now known to be a minor element of brain tissue.