It will depend on the size of the tank. That being said, a school of tetras / barbs for the mid level swimming area along with some small cory cats for the bottom of the tank, and hatchet fish for the top level of tank. The stocking levels will again depend on the size of the tank.
Make sure the tank is cycled before adding the fish and check the compatibility, that is water conditions, aggression level, size, etc. of the species of fish you want to keep.
