I have had my Koran ang Grey in the same tank in the past(no longer). The grey was a lot more aggressive at the end. I had to seperate them and now my Koran is in my 120. I hope he doesn't get too big for it. I am watching for changes in him. But the grey was mean not only to him, but if they started to go at it the grey would chase others also. Now that the koran isn't in there the grey has calmed down completely.