Ok, I don't want to beat a dead horse here, but water doesn't cycle. The bacteria that process ammonia, nitrites, and nitrates cling to surfaces. That's why sponges, or porous live rock are the best things to grow a colony of the right kind of bacteria. There may be some in the water, but not enough to cycle a tank.
Back on track though, what kind of fish do you like other than tangs? Are you looking for color, activity, pairing behavior, or what? 20 gallons isn't a lot to work with, but you do have some choices.