This discussion is going nowhere very fast.
You are not going to find anyone who tells you that having a yellow tang in a 20 gallon tank is a good idea.
There are many fish that would do wonderfully in a 20 gallon aquarium. Unfortunately, a yellow tang is not one of them.
Please check out clownfish, damsels, pseudos, firefish, gobies, some blennies, perhaps a small hawkfish, and other small fish that do not need a very large aquarium or who do not need a ton of swimming space.