Sponges are a great way to add aeration to small tanks. The little round Lee's filter is about the diameter of a 4 cup coffee pot and can be put under some gravel in one gallon bowls. While they're not powerful enough to circulate water on their own they work fine along with another filter which can be run for a few hours a day. If you get the small "professional" sponge it will run a 5 gallon on its own. I've had reliable fine filtration from these as well, the low flow-through makes them superior at polishing water.
For 5 gallon tanks I still advise either the Whisper3i with an intake sponge for single fish or an AquaClear 20 if you have fish+plants or several fish.