I have a 24" marble pim....he quickly outgrew the 125g at only 16-18", and this past spring we got a 265g. He is now as long as the tank is wide (the tank itself is 24" wide). I know in the next couple years when we move we will be building an indoor pond for him.
As for the arowana...I'd be more afraid of the catfish eating it. You wouldn't believe the fish, and the size my kitty fish has eaten. THeir mouths are VERY big (seen it when they "yawn")
and, consider the gravel/sand/ornaments...Had gravel, changed to sand. As of last night I scoped out all the sand that was constantly being swooped to one big pile on the one side of the tank....big catfish can move a LOT of stuff when they swim.