One way to really make any tank, not just a small one, looking very special is to add fish that school tightly.
From all the tight schooling fish suitable for a 10 to 30 gal. tank the following are the best:
- Ember tetra
- Boraras urophtalmoides
- Dwarf Indian ring loaches
The above fish are small and will create the optical illusion that the tank is much bigger than it is.
Also, for a different, more "rivery" look you can get some of the gobies that are fairly new to the US hobby - Rhinogobius wui, Black Velvet Sulawesi Goby, or any other Rhinogobius. These fish are real pets - they interact with you more than most other small sized fish. Also they lay eggs on a regular basis, which (combined with them being new to the US hobby) may make you think about breeding and selling them.
--Nikolay