How big is your tank? How long has it been established? Have you ever had tangs before? Do you have a easy and ready access to many different types of live and dried macro algae? Do you have a dedicated refugium for growing macro algae? Clown tangs, Acantuhrus lineatus
, are extremely aggressive and have very specialized living requirements. Their oxygen intake is higher than even the average tang, and they shouldn't be kept in aquaria smaller than 260 gallons. They are a much more free ranging tang than the average surgeonfish, and require heavy current to swim against or they'll go into "swimming frenzies" and bash their heads in. They will target specific fish and harass them until the fish is dead. The LFS
had one that lasted around two months in their 300 gal
show tank, and then it died. It was probably more of a oxygen quantity vs. water quality issue, because that show tank is just unbelievable, and they have many hard to keep things spawn in there. During that three months, it harassed the other fish so badly that it took three weeks after for them all to quit hiding in the rockwork. So adding it last really doesn't do anything other than give your other fish a chance. I don't know who told you that their clown tang wasn't aggressive, but I would suggest spending a couple hours observing said fish your own self. It's real easy for people to go "Oh, I had one of those once" but nothing shows more than seeing it for yourself.