What cichlids can i keep in a 20 long?

Convict pair or some rams
 
Your two best options, in my opinion:

South American: A pair of German Blue Rams, with a small group of hardy tetras and a few corys.

African (Lake Tanganyika): A few shelldwellers and nothing else. My favorite are Lamprologus ocellatus. Their personality is amazing and they ain't afraid of anything.
 
AquariaCentral.com