Love my little rainford goby. It's about half and half in the water column vs. chillin' out under the rocks time wise. Gorgeous fish. I'd highly recommend captive bred if you pick that one. I've heard a lot of the wild ones slowly waste away because they will only eat filamentous algae. But my captive bred one eats pellets in addition to sifting through the sand and picking the rocks for algae and he is super fat and happy.
I've got a pair of Banggais in QT waiting to go into my 60 gallon, and they're pretty neat fish as well. A little skittish, but for the most part they're out and about all day and get super excited when I show up with food. Definitely more of a water column fish than my rainford. A Banggai might eventually outgrow your 20gal though.