Go minimum 1200L (henwards tank size), fit a sump with 1-3 chambers with variety of filter media and put on an fx5 for best results - this should be enough. I run an fx5 and a 5ft sump on my 180g tank and it does a good job. + having 1 aro would be pretty boring, get large schooling fish (loaches look good - refer to loach master aka Henward for tips; however sharks/tinfoils/chalceus etc also look nice), once the aro gets big enough, add in some cichlids maybe? Indo tigers look good with aros too.
Another sweet combo you could try is a heavily planted tank (Plants are good especially if it spooks, as it will cushion it), and once the arowana passes 40cm (as well as keeping him well fed) you could try a LARGE school of cardinals (or whatever you prefer) as a small schooling fish). I have seen this effect and it is badass! The aro is not bothered with the small fish when it can just get a good feed from you - note: Aro must be of DECENT size for this to work, 20cm aro=bye bye little fish.
But a key point (in agreement with DavidR) is a wide tank, especially with silvers and blacks, room to turn is a must, the arowana looks much more comfortable and exploits better behaviour + it means a larger tank
Get bichirs as tankmates; they are awesome. :roll: