Here's a slightly different idea:
Look into pygmy sunfish (Elassoma species). You could keep a whole little colony of them in there. Add least livebearers (Heterandria formosa), pygmy killifish (Leptolucania ommata), swamp darters (Etheostoma formosa), freshwater ghost shrimp (Palaemonetes paludosus or P. kadakiensis), and some physid and planorbid snails for a sweet little mini-southeastern US biotope. Good and biotopically appropriate plants include Echinodorus tenellus, Eleocharis acicularis, Najas guadalupensis, Bacopa caroliniana, Azolla caroliniana, etc.
Swamp darters are common contaminants in ghost shrimp deliveries; you'll probably have to order the rest of the fish online. I can point you to some vendors if you're interested.