I just asked many people the same question. I have an angel fish I may have to move to a smaller tank. They said she would be fine in a 20 (but 29 or larger is recommended), but to do water changes once a week.
Also, they can be territorial/aggressive. I know mine is and that is why she is not moving into the bigger tank with everyone else. Alot of times they are okay with the fish already in there, but will not allow new fish into their tank.