That is just something you need to be careful of. Many sites or LFS will carry a fish that may be miss named, and this is one of the reasons we encourage researching before you buy. Owning the book Marine Fishes by Scott W. Michael can also help identify over 500 marine fish.
Calling it by where it may have been caught can also tell you if they used cyanide to catch them.
The fish in question can be called a blue tang, regal tang or even his scientific naem of Paracanthurus hepatus.
Countries where Paracanthurus hepatus is found
Amer Samoa ASM native
Australia AUS native
Caroline I. CRL native
Chagos Is. native
Christmas I. CXR native
Cocos Is CCK native
Comoros COM native
Cook Is. COK native
Guam GUM native
Hong Kong HKG native
India IND native
Indonesia IDN native
Japan JPN native
Kiribati KIR native
Lord Howe I. native
Madagascar MDG native
Malaysia MYS native
Maldives MDV native
Marshall Is MHL native
Mauritius MUS native
Micronesia FSM native
Mozambique MOZ questionable
N Marianas MNP native
New Caledonia NCL native
Norfolk I. NFK native
Ogasawara Is. native
Palau PLW native
Papua N Guin PNG native
Philippines PHL native
Ryukyu Is. RYU native
Samoa WSM native
Seychelles SYC native
South Africa ZAF native
Sri Lanka LKA native
Taiwan TWN native
Vanuatu VUT native
Wake I. WAK native