I would not keep a dwarf angel of any kind in less than a 75g, maybe a Pygmy like a Cherub or Flameback, but even that is pushing it IMO
. I know some sites will say its okay. I tend to be more comfortable with sites like liveaquaria that go bigger. IMO
, too small causes problems the majority of the time. Larger rarely does.
A yellow tang in a 30 is a bad idea...it really needs about four times that volume.