Get a half dozen Tanganyika shellies. Stick some coral in your filter, two dozen escargot or similarly sized shells in there, and find one of the Lake Tanganyika dwarf shell dwellers. Lamprologus multifascitis. Neolamprologus brevis or N. ocellatus are all fun.
Heck let it cycle for 6 months with pond snails and ghost shrimp, then add a male and 5 female dwarf puffers.
10 female bettas and 5 ottos.
8 diamond tetras, 6 panda cories, and a Clown Pleco
Set the filtration up to put a strong current end to end and add 5 hillstream loaches and a dozen or more white clouds or half that many pearl danios.
Get a pair of everglades pygmy sunfish and try to breed them.
10 pencilfish, a pair of apistos, a banjo catfish, and a mess of driftwood and boiled oak leaves (works better in a 40).
5 to 7 striped Loaches and 5 harlequin or espi's rasboaras. That would be a really active tank.
5 Skunk loaches (nothing else).
10 pygmy cories and 15 glowlights, green neons, neons, flame (the gold von rio variety is quite striking), or silver tipped tetras. or 10 tetras and 5 ottos.
8 Tiger Barbs (nothing else)
6 kuhli loaches, 2 male and 6 female Cherry barbs.
12 to 15 threadfin rainbows or Pseudomugil gertrudae.
A convict and a half dozen zebrafish
3 albino cories, 1 albino bristlenosed pleco, and 10 albino neons.