I agree with Olympia. A heater really is a good idea. I even have one with my goldfish tank. I highly recommend one even with fish that like temps in the lower 70's and upper 60's. It will also open up a lot of possibilities.
For finding your water hardness, if you can't find it online it will probably be best to call your local water treatment plant. They will have the dGH and dKH or ppm written down.
Well the good news about hard water is that most tetra don't like it. All of the livebearers are hard water. In a 20 long you could have males and females. Guppies are a bit overdone, but endler's livebearers are rather unique. There are some goodeids
that might be able to fit in a tank that small. The redtail splitfin
could. My favorite the tequila splitfin
might be able to fit, but they are rarely found in captivity because they are critically endangered in the wild. Goodieds are also known for not eating their young.
Dragon fish, sometimes called violet goby or dragon goby, need rather large tanks as they get to be quite big. Seriously fish says 48" or longer. Gobioides broussonnetii (Violet Goby) - Seriously Fish
I'd kinda like to have them in a tank with black mollies and archer fish one day. /dream
Cichlids are also hard water fish, but I know basically nothing about them.
Depending on how hard your water is, some cory cats could work. Most loaches (Cobitidae) that I know are soft-water fish. You would have to research which ones are specifically hard water fish. Some of the hillstream loaches (Balitoridae
) might be. I've never kept them, so I'm not familiar with them. But if you do decide to go with hillstream loaches, you could look at a riverine tank setup.