You could always buy a nano-filter. They do about 20g/ph or something rediculous. That and small water changes every 3 days or so. Here are some tanks that emphasize the word 'Micro'.
Ive always said it takes more skill to make a nice looking small tank than a nice looking big tank. I have seen some pygmy rasboras that to the best of my memory got to be about 1/2" long. Id do trio of those, and maybe a couple of cherry shrimp.
Where I am, as the heating is off whilst I'm at work, the room temp varies alot even just in one day! It will go up from 17C to 21C for 7 hours then slowly back down over night. As there is so little water to retain the heat a little tank might fluctuate IMO.
Love those NANO tanks, will be good if you can pull it off