Thanks for all your concerns. I have a silver arowana by the way.
I decided against the tank. It was going to cost me $200 for a tank and a stand, so I decided I'm just going to build a little plywood pond in my garage, either 6'x3'x2' or 6'x2'x2', since it will be cheaper that way and bigger. I'll be getting the pond in 5-6 months at the latest, and my arowana only grows about an inch a month. I expect he'll be around 20" by the time he moves to a new pond; a 20" arowana in a 6x3x2 seems fine to me.