New fish for dummies?

[StudentoReptile]

So I am leaving my current job, and part of my duties was the office aquarium care-taker. It's a 52-gallon cube. There's a subadult Jack Dempsey in there that I am rehoming because, obviously he is getting too big for it.

My inclination is a small school of ONE species of fish, because I keep this tank as simple as possible for rest of the staff who will taking over its care. Any suggestions for a dummy-proof fish that would do well in a 52-gal cube?