i dont think arrowana are brackish, or silver dollars (the red hook, nice fish) for that matter. id get them in a FW setup. they prefer acidic water, a pH of 6-neutral. brackish is generally around 8, plus they'll need a bigger tank as they grow.
it is a nice tank though. get some sort of substrate in there, it will look even better. i use playsand in all my tanks. give it a wash through, and your good to go. its cheap too. i get 15Kg for £3. that gave my 30gal a 1"-2" substrate. you would need about 3-4 bags. if you decide to add some.
maybe add some wood too, but it can lower your pH if it releases tanins, and most new wood will. so you can do waterchanges, or keep it in a bucket for a few weeks, changing the water every few days. that will get rid of most of the tannin.