A 20g tank dramatically limits what you can put in it, as it's too small for many single fish. Additionally, other species which, by themselves, would fit in a 20g, won't be healthy or happy in less than a school of 4-6, which might be too much for a 20g.
Then there's water parameters. Rarer fish are generally more difficult to keep healthy, and might require specific pH/GH/temperature levels which many, or most, are unable or unwilling to keep up with.
Rather than ask what you did, I'd suggest asking for suggestions for fish given your personal standards: tank size, level of maintenance you're willing to keep up with, whether you'll want plants in there too (many fish are *not* plant-friendly), what you like in fish (color, size, activity level, etc.), and anything else you can think of. Oh, and how much money you're willing to spend.