Oscar the guard fish
Oscar vs Felix
While all fish have personalities, only a few fish have enough personalities for people to regard them as pets. For saltwater, tinker's butterflyfish and dogface puffers may come to mind. For freshwater, oscars clearly fit the bill. A big part of what makes an oscar a "pet fish" is their eagerness to interact with life outside their glass box, and not just with their owners.
Take cats for example. Cats are notoriously interested in aquariums. Oscars are notoriously interested in protecting their aquariums. When you mix the two, you get some entertaining results.
Oscar KOs Felix in under 30 seconds!
An adorable kitten is lucky there's a pane of glass between him and what he thinks is his play toy:
Who's stalking who?