tang not looking well

[rickjames]

Sounds like the fish is stressed, as has been mentioned could be the fact that it is in a 20G tank (which at the VERY LEAST is contributing to the problem), or could be a water quality thing. How are N02/N03/NH4 levels? Maybe a lack of oxygen too, are you running a skimmer on the tank?

I won't give you a lecture, but having that tang regardless of size in that tank is probably the root cause of your problem.

Good luck.