This is because your arowana is dying from ammonia poisoning. Did you tell the fish store that you have a tank full of ammonia? Nutrafin does nothing to neutralize ammonia, prime does detoxify ammonia. You need to change enough water daily to keep your ammonia and nitrite levels below .25 ppm, did you buy a test kit? As for what bottled bacteria to use? Start another thread asking that advice, I have never needed to use it so I can't really comment.
Best thing you can do until you get what is required from the fish store is to do daily 50% water changes and hope for the best. If you don't act now the only update you'll be giving in a week is of your dead arowana.
