more than likely two arowana's would fight, you would be very lucky to find two that would live together. I have a 8ft 2f.5ft 2.5ft tank once your Arowana has grown to over 2ft & is very wide it will look great in your 8ft tank.
You could get other fish like flagtails , tinfoil barbs, tiger fish , stingrays.
If your fish has been on his own for a while in your tank he will of established his territory & will fight with your new Arowana which would more than likely kill the fish from stress
