return that fish!!! it gets HUGE!!! more than double your aquariums width. if you keep him, you'll eventually start waking up everyday with one less fish. they're really cool fish, but they pretty much need a pond.
after you've gotten rid of that guy. i would say may polypterus endlicheri endlicheri, polypterus ornatipinnis, polypterus delhezi.
I personally really like polypeterus senegalus, because they have the most personalities. But, most people don't like them cause they're pretty common.
besides polypterus i feel like you could go with a few "foot-long" south or central american cichlids. maybe not something really aggressive (like a dovii), but more friendly like oscars, Jack dempsey, green terrors, etc.