One Senegal Bichir
OR
one Arrowhead Puffer
OR
one Cuban Cichlid
or maybe a Loach pack? You could put like 6 or 7 Yoyo Loaches, Striata Loaches, Kubotai Loaches, or Dario Loaches in it, or two of each.
Or an asst predator tank. Example
1x Nandus nandus, 1x Leopard Ctenopoma, 1x African Butterflyfish, 1x Peacock Eel
Or a gigantic Exodon swarm... like 30 or so, if you can afford it plus the filtration.