You could do cory cats, ottos if it's highlight; most livebearers would be fine. Most anubias, Java fern and Java moss as suggested would all be fine as far as plants go. Your plants are heavily dictated by the lighting you have, if it's a fluorescent bulb like the ones that come in kits then you'll mostly be stuck with the ones listed. Fortunately those plants are hard to kill, they can be planted minus the moss, or tied to driftwood, rocks or even floated if it tickled your fancy. If u get multiple pairs of livebearers you could end up with an excess of fry. Tetras, dwarf cichlids, pretty much anything including the parents will consume the fry. Plants will hide them especially java moss, and any livebearers will probably spawn with relative ease. Ultimately depending on your plants and filtration you can get away with a decent number of fish, but lighting will dictate the plants you can have and to an extent how many. With a little more info we could help tailor a stock and plants to your interests, but ultimately you have a lot of options even in a tank as small as 20 gallons.
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