The fish you have is a black, or what is known as a jardini in some areas. That species of fish get very large, so you guys have two choices, return him or get a bigger tank, I have done the research on housing any species of arowana, here is the link for where i am buying my aro tank,glasscages.com, these tanks are ALMOST ideal for housing aros, particulary silvers and blacks,go for the 240 gal long, or the 280gal. I will warn you the fish you guys have is NOT for the beginner aquarist, but with the help of the online community and the many books available you can keep it correctly. I highly advise you take rallysman's
advice and go to monsterfishkeepers.com's arowana forum though for more users who have had or currently have arows.
Thanks for coming to us for help, I knew someone who did the same thing, but didnt ask anyone about it, and they lost the fish, ME, i regret it every time I look at an arowana, but I learned what to do and what not to do