Dude that tank is amazing. Very interesting shape by the way. Sorry about your arowana, bummer losing such an awesome fish like that.
Personally, I really enjoy aquascaping tanks. With a tank like that I would layer it with a dark gravel/sand add some smaller rocks and a huge piece of driftwood as a centerpiece. Maybe even some plants, maybe they wouldn't get bothered because of the sheer tank size? And a nice DIY 3-D background.
Are you planning on adding anything else in the near future? A school of geos would be pretty neat. They'd probably get eaten though.
Cheers,
Art