I agree with what the info most people wrote. Ideally the bigger the tank the better 150+ for smaller tangs. Adding them at the same time definately helps. Keeping tangs from two different genus's, shape, color, and size. As an example I have a Ancathurus(Powder Blue)that is about 4 1/2". I wanted to add a Zebrasoma Rostratum (Black Tang). I knew that getting a larger Black Tang would help to alleviate aggression b/w the new addition. The Black is a different color, genus and shape. I got a 7" Black and still the PBT gave him hell for abot ten minutes until the Black Tang out him in it's place. They get along fine now. Be aware that there is no golden rule with compatabilty with tangs or any other fosh for that matter. Make sure you have an alternate plan in case thing s don't work out. Also If you add another tang after having one, keep the existing tang well fed, add the new tang with lights out, and if possible change the landscape of LR just before adding new tang. Hope this helps. Here's my two tangs. Regards, Tim