Adding more fish to quarantine tank

[Myka]

Wait until the Tang is done his quarantine before adding any more fish. Patience in this hobby is very important. You don't want to risk bringing in disease to a fish that, I assume, is doing well.

In the future, a 3" Yellow Tang and other fish of similar size and activity level would fare better in a 36" quarantine tank like a 40-gallon or a 33-gallon. You may find you get greater losses by using such a small tank for a fish of that size and activity level. A 20-gallon quarantine tank is better suited to smaller fish, or a small group of Chromis or Damsels like you're planning.