Now, that depends on what you mean by "fish". If we're talking tiny fish like minnows, anything can eat that. Dolphins--likely the most favored aquatic predator by humans--eat slightly larger fish like mackerel. Larger fish like tuna and bass prey on fish of similar proportions. And at the top are orcas (aka "killer whales") and sharks--my personal favorites. The predators at the top also prey on animals like seals. They do not prey on human beings.