Just to make sure on the water...if you could post the GH and pH while you're at it, it will help.
But moving ahead, your options are more than you might think. Nano fish are very popular these days, and you could have quite an active and thus interesting community in a 20g with the dwarf rasboras. Beyond those, thinking of more "standard" small fish like the tetras, rasboras, pencilfish, dwarf cichlids, most of the smaller gourami (the Honey Gourami is not the only one when you have soft water, there are several others even smaller), some of the small rainbowfish... .
Do you have or intend plants, even if just some good floating species? Floating plants make a big difference on all of these fish mentioned.