I have a 20 long with 4 female betta, 9 neon tetra, and 6 zebra danio. I use just one medium sponge filter and take the plastic tubing off the top. Having bettas LOW FLOW is a must, so No HOB for me, works fine, waters clear. (Medium is a little tall for the 20 long), but id rather use one medium sponge filter than two small ones. Id say go with your personal preference. A small or nano sf would definitely have a much lower profile and hide easily behind plants. But you would need two of them, or one small sf and one nano hob filter in addition would be fine.