What are you not happy with, the type of sponge filter or the way it filters?
I experimented with a 10g for several months with no filter; well-planted, a heater, and fish consisted of Corydoras pygmaeus (9), Boraras brigittae (9), 2 shrimp, a dwarf puffer [puffer and shrimp arrived unintentionally with the corys], and Malaysian livebearing snails which the puffer kept well controlled.
This worked, though the water was not truly clear, so I added a Hydor sponge filter. I now have a single Elite sponge which is less noticeable.
The clarity aspect of the water is the benefit of a small sponge filter, plus water circulation. If you decide to go filterless, laying the heater (submersible obviosly) horizontally near the substrate along the back wall would create the best thermal convection currents. I would do this before using an airstone.