What a dead body becomes after it is creamated; not all that unlike, "worm food" which is what a dead body becomes when it is buried in a graveyard.
It's called fish food because the ashes of dead people are frequently sprinkled into sizable bodies of water such as lakes, harbours, channels, sounds, bays, oceans, etc.
{Aaron): Hey Jimbo, didjya hear that Bernie in Juneau got hosed down last weekend?
{Jimbo}: Yeah, he became fish food a couple of days ago; I think that the video of Bernie's parents sprinkling his ashes in Gastineau Channel is on YouTube by now -- if they have YouTube in Alaska anyway.
noun: A ridiculously slow method of eating food that is implemented when terribly hung over. It utilizes the pulling apart of one's food into small fish-food sized flakes before being able to eat it.
"Man, J was so hung-over that he was rocking Fish Food Technique at breakfast. It took the dude 2 hours to eat a bagel."