I would say expand on the Clown Loach and Congo Tetra theme, go for a full-on African river biotope!
- Butterfly fish
- Alestes Tetra
- Yellow-tailed Congo Tetra
- Distichodus species (single as not social?)
- Bushfish
- Filter Shrimp
- Debauwi catfish
- Lionhead Cichhlid
- Thomasi Dwarf Cichlid
- Spiny eel
- Transvestite Cichlid (? Only reach ~5cm but they are supposed to punch above their weight. Might leave lasting impression on others if everything bought as babies, so as adults they remember this fiesty little fish)
- Syndontis catfish
- African Barb
- Harlequin shark (single specimen, my fave LFS has a tank of these, but not researched them yet... WOULD NEED GOOD COMPATABILITY CHECK, as can be bullies and they grow to ~15cm
- Knife fish
- Tilapia
- etc. etc.