You dont habe velvet i saw your pic zoomed in.. i can count and see the spots.. its just a really bad case of ich.
That fish will die if you dont start hyposalinity on it. I can vouche that the yellow eye tang can handle hyposalinity at 1.007 just fine.. thats what i kept mine at for 2 months.
The fish csn also handle the drop ommediatly so put the tank at 1.007 to 1.009 and in 3 to 7 days when the ich falls off it wont come back.
If this were velvet hed be dead already. Some people on the forums cant tell the diff between ich and velvet. Sometimes i cant either.
Does your kole tang look like this ?.. if so thats ich not velvet.. little hit when taking pics... dont use all blue lights in your tank..