Any of the nano fish would be great in this tank. I actually have two of those cubes (the Fluval 7.9 gallons). I have a betta and pygmy cories in one and a betta and kubotai rasboras in the other.
You could get a nice school of nano somethings. Ember tetras, kubotai rasboras, pencilfish, celestial pearl danios. You could do one or two sparkling gourami even with a small school of other nanos- pretty little fish with big personalities.
Bettas: Roland, Belle, Snow White, Polo, Elise, and Titan. | 120 gallon community tank | 55 gallon angelfish tank | 20 gallons of guppies