There's quite a few options for a 20 gallons. Just to mention a few, you could:
do a shelldwellers species tank (if your water is hard)
or keep a pair of rams (if your water is soft)
species tank with puffers
or go brackish with bumblebee gobies
Lots of killiefish to choose from.
Any of the following appeal to you?:
bottom dwellers: corydoras, khuulie loaches, bristlenose (only 1 in that size tank), otos, clown plec
schoolers: neons, glowlights, black neons, black phantoms, harlequin rasboras, espei rasboras, serpae tetras, flame tetras, ember tetras, cherry barbs (though they don't have to be in schools)
livebearers: platies, guppies, endlers
feature fish or trios: dwarf gouramis, honey gouramis, sparkling gouramis
oddballs: peacock gobies