Try a pair of shell dwelling N. brevis cichlids. There are a few variants available. They keep a territory of just 4 square inches or so. This is one of my favorite species, very pretty with many interesting behaviors and fairly easy to breed. The fish will be perfectly content in a 5 gal tank as long as you have some proper shells for the female to choose from, a diggable substrate, and feed them a good varied diet. I kept an algae eater with my pair and had no issues. They are quite peaceful.
I believe they mate for life, so be certain to get a bonded pair.