A group of 3 or 4 Leopard Bushfish could live in there for life, they slowly reach about ~14cm in the hobby (wild ones reach ~20cm), my 3 bought as youngsters 3 year years ago have grown from ~2cm SL to ~8-11cm SL (the biggest one, possibly a Ctenopoma occelatum, has now exceeded the size of the fourth one I added when these guys were ~6cm).
You could add other "African oddball" fish, mild mannered as unaggressive feeders such as...
African Butterfly Fish
A group of Synodontis nigriventris
A group of small deep bodied tetra such as Yellow Tail Congo Tetras