There are many interesting and beautiful fish native to the US that can be kept in unheated tanks as small as yours. For Pygmy Sunfish a 14g is a big tank, but they should be kept as the only fish in a tank. You could have 25 or 30 adults at any given time in a tank that size, (although likely only 2 or 3 males would claim territories and color up all black and electric blue) along with plenty of babies. I challenge anyone to show me a fish with more personality than a Swamp Darter, and you could keep several in that tank, along with other fish. Depending on where you live you could have true aquatic gems right below the surface. There are also a couple of excellent commercial suppliers of native fish, and lots of nice stuff comes up for sale on AB.