Like jmac says, I would first think about my water parameters and take it from there. I have hard alkaline water so what I keep in that size tank is some kind of livebearer (guppies get on well with platies, or some shops sell less common species such as limias) and corydoras catfish- they prefer a sandy substrate though, and shrimp. Small rainbow fish might also be a consideration or galaxy rasboras. Some tetras are also hardy enough to cope with this kind of water: x-rays, black neons, black phantoms.
If the water was soft and acid, I would go for a shoal of tetras or harlequin rasboras and corydoras as bottom dwellers.