Impur is right, tangs cant go into just any tank. The problem is that most LFS sell these without giving much concern to the new reefer. They see a small sailfin or nice little "Dory" (Regal Tang) and buy them for their home tanks.
They dont realise that these fish grow incredibly big and are open water swimmers. If anyone saw tangs in their wild state they would realise that they are not like clowns or dwarf angels that have small territories, the territory of a tang ranges for miles!
Regal tangs and other similar shaped tangs are probably the ones needing most space (anyone seen the space a Sohal tang requires?

)
Yellow tangs and similar shaped tangs require slightly less space IMO but i do mean "SLIGHTLY" Tangs should not even be attempted in a system less than 75 gallons (UK) IMO.