1. How long has the tank been up?
2. How big is the tank?
3. What is your filtration?
4. What is the size of the arowana?
Ideally you want the tank to be 4 times the length of the fish *in this case the tank would be about 10 feet), 2 times the length of the fish for the width (in this case it would be 6 feet), the height can usually be 2 feet with no problems, so that makes for the tank to be about 900 gallons. You need to consider filtration, for a large fish you want 2 times the amount of filtration the filter is rated for (so if you have one rated for 400 gallons, you want 4 of those filters to filter twice the volume of the water, though this is just my opinion), I personally would recommend a few sponge filers in the corners (if possible) to aerate the tank and to remove any detritus at the bottom and have a few canister filters (eheim or fluval IMO). The tank should be well cycled (which is sounds like it has been), preferably for 3 or more months, and the pH should be around 6.5, but anything from 6-7 would be acceptable. I'd get the arowana and make sure the water parameters match up very closely before adding him in, and if possible quarantine and medicate the arowana first, preferably medicate for 1-2 weeks and quarantine for 1-3 months if possible