Hi all,
It is not a very aesthetically pleasing option, but even for a nano I use a maxijet 400 powerhead with a really big ppi 10 sponge. For the little tanks I use the 12" x 4" x 4" black sponge blocks from Kettering Koi, siliconed to a plain white glazed 4" tile. I cut the sponge block so the surface of the sponge is just below the water level, and then I plant the top of it with moss or any small carpeting plant, it sits in a corner and pretty soon a good level of biofilm, moss etc grows on the sides of it as well. It is really acting as a "Hamburg Matten Filter", and I only squeeze them out every 6 months or so when theflow rate has fallen dramatically. Shrimps (and fry) love them as a feeding surface and they offer a huge amount of biological filtration.
cheers Darrel