A 2-3 Foot fish needs alot more then a 125g tank.
Black aro just as their cousin the silver aro deserve to be kept in at least a tank that has a length 3x of the arowana and a tank width at least equal to the aro's length. Figuring the arowana will get to be 30" long it'll need a 96x30x24" 300g tank, possibly bigger as it can grow beyond the 30" length.