I guess by aggression you mean a glass banger? You did want a solo fish? I also agree with the severums,a few kinds to choose from.I love my chocolates too.I once saw a large black belt (vieja) it was big as dinner plate.I think a few kinds of vieja could work.
Stay away from true parrots imo. Very timid fish and need super prestine water. How about a severum. Red spot severum get decent sized. They are peaceful. And deff an eye catcher.
I have severums in both my tanks, 125 & 150g. I have red, gold, green and turquoise in my tanks. They grow to 8-10", very attractive and peaceful fish.