Things to consider. Arowana need to be kept in a tank that is at least 3x their length and have a width/depth at least equal to there length. So your tank is good for a 18-20"(Assuming Standard 6 foot 125g)
You live in the US, so only legal aros are 2 from South America, 2 from Austrialia and on from africa. Asian aros are illegal in the US.
The african aro is a filter feeder when young and should only be kept by advanced aquarists. The silver and black will get near 3 foot with proper care. Silvers grow quite fast 1-2" per month blacks do grow a little slower.
The ausies, Jardini and leichardi will get near 30" but are not as flexible as their South American cousins, so they still have the same tank requirements. Hopefully others will chime in with growth rates for these guys.
To sum it up you will likely need to move an aro out of the 125g within a year, possibly a little sooner or later.