My experience with it (RTG) is, if you had other fish in there and introduce the aro later it would be fine. Feed it when it's hungry or else everything in there (smaller fish)will become dinner. I had angle fish, parrot, bichir and sting ray before i introduce my aro and it's fine. But when i recently put an ST in there it tried to make lunch of it.
Not too bad IME, i have 6 clowns with my 18inch RTG, it never goes near them, never tries to eat them. Even if i'm trying to starve it on to pellets.
It goes after my Dats a little bit but it's only a one bite attack.
That's what i know.
My rtg has been quite agreesive in the past having killed 5 geos a 13"plec,a 6" dat and a 17" gar BUT since i put the tinfoils and the big dat in he has calmed down alot but still attacks the tinfoils,but i dont think hes trying to kill them because he attacks them and gets them in his mouth for a split second but then lets go and goes after all the scales that have come off,and he will have a little dig at the dat now and then but he deserves it as he always nips at the rtg and the rtg does not EVER go to attack my bichirs,they must smell bad as he always recoils backwards when he has a sniff lol.