Alright the tang guy is here...
I agree with what's being said, a tang is not an appropriate choice for a 20 gallon tank except perhaps as a quarantine. These fish graze over vast distances of water in the ocean and it's understandable that the more they can mimic their natural behavior, the "happier" they will be. While we cannot measure stress in fish (or at least at the moment its cost prohibitive and senseless due to harm to the fish), the best we can do is go by their behavior in the wild and their different behaviors in different sized tanks in captivity and draw inferences from these observations.
One thing I can tell you for sure, is that a happy tang will love to slowly cruise their territory in what is hopefully, a long tank. IME the minimum size for small zebrasoma tangs is 55 gallon. This is 4 feet of swimming room. A happy tang will have great color, feed well, etc.
An unhappy tang will often zip around the tank side to side, make quick, abrupt movements up and down, and may even show color loss or "stress colors". They're more prone to disease in a smaller tank due to stress as well.
With all of that being said -- while I agree with many of the posters, this is not how I envision us as a forum communicating with one another. I disagree with many things I see on here, and many probably disagree with my assertions as well. Part of what makes this forum so great is that we DONT patronize and demean people that we disagree with. Some people simply don't know. I'm very passionate about tangs and in many cases have been called the tang police but I try to approach people from a point of understanding and helpfulness rather than belittling and sarcasm. Let's all try to keep personal attacks and rude comments at bay and discuss facts or even conjecture in a professional and kind manner.
To the op, I'm sorry, this is not how this community generally treats questions. It is, however, something that many aquarists are very passionate about and so it is more likely to elicit this type of behavior than other threads, hence the "tang police" jokes many have heard.
Happy reefing!