There's not many fish that you could keep in that tank as it only works out at about 4 gallons. You could probably keep some Golden Apple Snails but other than that, I'm not sure.
Are we talking totally coldwater or cooler than normal tropicals say mid 60's'F
If slightly cooler then White Cloud Mountain Minnows (WCMM's) Tanichthys albonubes
The problem lies with the size or volume of aquarium you have,
There are loads of nice obscure colwater fish such as Cyprinella lutrensis but alas is way to big for that tank.
Regards
BigC