I have been wondering the same thing. It really depends on the size of the arowana. When that trap door opens, it is a cavern in there. Right now I am not willing to risk my Pbass. My arow eats goldfish just larger than my smallest pbass, so obviously I won't be experimenting to find out. I have an empty 250 that will become home to some of them.
p.s. I got both my pbass (15 of them) and my arowana from JuanTamad. He has some really amazing fish and some awsome racks of tanks. I was almost speachless when I was there.