Originally Posted by Quantum
Tanichthys albonubes - White Cloud Mountain Minnow - could work as long as it doesn't get too cold
I think they need at least a 24 inch long tank as these guys are active swimmers. But they are some of the only fish that I would suggest for a small unheated tank. Three or four male guppies might also be able to work depending on the air temp.
The problem with not wanting a heater is that unless the room stays within the species' range you will have to cool or heat the tank at some point in the year. I even have to heat my goldfish tank (a species commonly touted as good candidate for an unheated tank) in the winter because the air temp simply gets too cold for them. Maintaining a tank without a heater is rather difficult. It's more difficult than maintaining a tank without a filter.