how big is the yellow.this guy is the problem they are very aggressive to other tangs.i have gad 5 tangs in a tank.all got along but its the size and adding order that really helps.
dont put the yellow in the new tank before another tang.add at the same time and make sure they are at least the same size or the new tang is a little bigger.you could add a bunch of other kinds.unicorn,kole,naso,chocolate,orange shoulder,achilles.usually no problems with the hippo since they are a mello fish and dont have the same body shape as most other tangs.
i had a hippo,purple,unicorn,achilles and sailfin.untill my nasty bout with some super ich.sucks because i had my purple for 6 or 7 years.i just couldnt get a handle on the ich.it kept coming back for well over a year.3 hypo and one copper treatment.in that span.