Hey, we were just in the Kuhli loach thread. Guess we both love our bottom dwellers .
I have a Clown Pleco in a 37g tank and that is the same footprint as a 20g long. I also have 2 Albino BN in my 20g long tank (Juvies.) You should have no problems keeping a Clown in a 20g long. Just provide it with some driftwood to munch on and feed it veggies and Algae Wafers.
Man my Pleco's wont get off the drift wood! I Got veggies in there and they wont touch it - they just love the drift wood! The Snails go crazy for the Zukine!!!!!