IMO livingart and the obsticle are under-estimating things. A properly cared for silver arowana should reach 24"/60cm in a couple of years, and most will keep growing to ~30"+. To adequately house it you're going to want a tank that is at least 3x the length of the fish long, and as wide as the length of the fish. a 6x2x2' tank would be an adequate temporary home, but long term you will want something with a footprint more like 8x3'.
Here's a couple of pictures to show you what you're getting into, arOwana are amazing fish, but caring for one properly is a pretty major undertaking.
Taken at the Berlin Aqurium, the largest silver in the display was over a metre long and getting close to 30cm top to bottom.
And here's a video of a guy from MFK's tank with several large arowanas, making an 8'x36"x30" tank look small...
http://youtu.be/UvwQTh0OTYI