Do you mean the name of the tang? Or the wrasse (( Labroides dimidiatus ))?
There are a couple of issues with keeping them in our tanks;
1) Removing them from the reefs (( ocean )) takes them out of it, where they preform valuable service --- much more valuable in the ocean than in our tanks.
2) While they do still clean in our tanks, the food supply is limited, unlike in the wild.
3) They don't often take to prepared foods, slowly starve to death.
4) They can often end up annoying the fish in our tanks --- always looking for parasites to eat.