There are only two species of arowana in Australia; Jardini (sometimes called pearl) and Leichardti (sometimes called spotted) arowana. I'm not very confident at giving an ID on young australian arowanas but jardini tend to have a more curved back (head points downwards) and a leichardti has a more straight back (head points more forward without a large noticeable curve) when adults. Additionally, although I don't know if it's true, leichardtis are said to have shorter barbels compared to Jardinis.