This is your tank do whatever you feel like. I've kept silver and other arowana in 6' tank no problem. People exaggerate about the fishes full size. Some will never hit that in captivity or even in the wild. Your tank is freaking huge. Probably one of the larger ones and at that size it can house A LOT of fish. I've seen people keep huge Pacu in 240g seems super happy and lived just fine. A LOT less space than yours. I've had motoros that thrived in my 210g before I sold it. Anyways point is you have a GREAT tank get whatever you want. Fill it up go JDM man