really depends on the fish, RT's do seem to just be plain mean. Yet, I have had my GT with multiple other fish,both aggressive and non aggressive and he has always left them alone. I heard this a long time ago and it might be a wifes tale .I was told if you lower the temp to the lower side of their natural water parameters that they arent as aggressive. I do this, slowly, if I see aggression starting up. Also keep them well fed, have lots of things in the tank to break line of sight and have a big enough tank. Overstocking wont do anything except kill fish (which should be enough not to do it) and waste money. If you cant keep a large enough tank for the fish you want, be prepared to change your stocklist to something for suited to your situation. Not trying to be harsh or smash dreams, just trying to be informative.