Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Look also into any fish that can handle that low temperature, but the small tank size is a problem, as is the lack of water movement.
Some tropical fish can handle temps into the upper 60s F, I do not know if they can go any colder, though. Certain Cories (Pepper, for one), Platies, White Cloud Minnows (you already have these), Certain Barbs (may eat plants), Zebra Danios, a few Tetras.
Some Loaches thrive in cool water, and are actually from temperate regions, not tropical. They get way too big for your tanks, though, and thrive in faster moving, highly oxygenated tanks.
Goldfish (of course, they get too big, too, and they sure eat plants!)
Continue looking into other fish that are native to temperate zones, not tropical.
Look into Planet Catfish. Find any species that is OK in the temperature range, then click on a search feature that says 'Find more like this'. There are ways of refining the search, but I have not done it much. Just go there and look around. Of course this is only catfish, but there are quite a few that might work, if you can arrange a slightly larger tank (perhaps a 20 gallon long).