Ok... I'm not trying to sound like the tang police here. That fish cannot be housed long term in a 20 gallon tank. Odds are that your fish is not going to live and here is why. The myth out there is that fish only grow to the size of their environment. This is completely false but it may be viewed, at best, as a partial truth for some fish.
What you have there, and correct me if I am wrong, is a Naso brevirostris. They grow to a wild size in excess of 20 inches.
Tangs, as far as I understand, will grow to the size of their environment. That only applies to the outward appearance of the fish. Their internal organs, however, continue to grow. This essentially results in a slow, painful death. The fish appears to be perfectly healthy to the aquarist and then all of a sudden takes a turn for the worse.
I cannot guarantee this is what is happening in your case. Odds are, though, that the fish is terribly stressed because that tank isn't nearly big enough. The stalking behavior that you described is because he just didn't have enough room in that tank. I am surprised that you have been able to keep him alive this long in a tank that size.
