Instead of 5 of the same tang, you could perhaps put a variety of them in. My uncle has a Sohal, Saifin, Yellow, and two hippos all in a 150 and he has no problems. I've also had a Powder Brown and a Kole together in a 70 and right now I have a baby hippo in with an Atlantic Blue but they're separated because the Atlantic Blue has been all over the hippo. Also currently, my dad has a Naso, Yellow, and an Orange-Shoulder Tang together in a 70. I know the tank is not large enough and don't worry because they will be moved soon. I would advise that you put different species of tangs in together at the same time to lower agression and not allow one fish to become established as my Atlantic Blue has become. Well thats my story so Good Luck!