A school of mollies seems like the best bet. Get one male and a bunch of females if you want to have some fry to sell. In a 20 they will do great and they are long lived if you keep the nitrates low and add a bit of salt to the water.
My favorite black fish is Stomatepia pindu, a mouth brooding Tialpine endemic to only 1 tiny lake (Barumbi mbu) in Cammeroon, but at an adult size of 7" and very active a 20 gal is way too small.