Remove the tang or CBB and put him in the sump for now.if I were you I would find a home for the tang or butterfly,which ever you want the least.also you didn't give any tank info.how big is the tank?what fish are in it?CBB' and yellow tangs usually will fight,even more so IMO when adding the butterfly to a tank with an exsisting yellow tang.the tang is territorial and sees the butterfly as competition for his food.FWIW I would pass on buying such a large CBB since they have a horrible track record in captivity.I like to find small CBB's in the 1.5-2" range,that are fed in store before they come home.QTing the fish also helps them transition to the display since you can get them eating good before introducing the fish to the display.