silver or black arowana in polypterus tank

Hey guys. Im a poly fanatic who is looking to pick up an arowana. I decided the tank could use a "swimmer." Anyway, how long can I house a black or silver juvenile purchased at 6" in a 180 before I need to upgrade? I have plans to purchase an enormous tank in 2 years when I graduate and get a new job. I have heard they grow at rates of up to 2" per month. Is 2 years realistic, or should I hold off for another 10 months or so?

Also, has anyone ever kept large polys with large arowana? I have heard they work well together.

Thanks,
III

Did its presence affect the polys in any way? Did they hide more often, I mean to ask. My 2 ornates and faranah lap are just now becoming friendly with me. I would hate to drive them right back into hiding.

Did its presence affect the polys in any way? Did they hide more often, I mean to ask. My 2 ornates and faranah lap are just now becoming friendly with me. I would hate to drive them right back into hiding.

Also, what size tank do you have?