catfish....
You didn't mention what kind of fish are already in the aquarium, how large they are, or how large the aquarium is. This would be helpful information.
The African Brown Knife (Xenomystus nigri) is generally a secretive and inoffensive fish except to other knife fish. They are, however, predatory and will eat any fish small enough to fit into their mouth. That being said, the African Brown Knife is an excellent choice for a community tank with appropriately-sized tankmates. I currently have one in a tank with medium-sized (6 - 9 inch) bichirs and a number of Bush Fish (Ctenopoma species) and there have been no problems at all beyond the fact that the Knife took an initial disliking to a male Microctenopoma fasciolatum which it chased every time it spotted the other fish. This behavior lasted about two weeks and I haven't seen any problems between the two ever since.
Just be sure you don't accidentally get one of the larger African Featherfin Knife (Papyrocranus species) individuals by accident. They can easily be distinguished from X. nigri because the Featherfin has a dorsal fin that the Brown Knife doesn't have. Papyrocranus can reach 30 inches or more and has a well-deserved reputation of being fierce predators and are usually completely intolerant of other knife fish.
-Joe