Stressed Yellow Tang

[naesco]

Lampshade, Tangs require 6 foot tanks. Your tank is less than half of what they need.
Remove the blue tang (which gets the size of a pie plate) and return it to the LFS as soon as you can.
Make arrangements to re-home the yellow tang until you upgrade to a tank large enough to accommodate tangs.
There are tons of other fish suitable for a small tank that you can choose from.