Swimming up and down is a bad sign. The fish is likely stressed and instinctively trying to get out of the tank / bad water. Relaxed arowana swim horizontally, mostly at the surface.
If I had to guess, your tank may not be cycled and/or your water parameters are not stable, swinging too much.
Or maybe it's getting bullied.
Or, if you have been feeding it only shrimp/prawn, it is suffering from vitamin B1 deficiency.
Or maybe it is feeding fine and you are trying to overfeed it.
It is conceivable that it sees its reflection and trying to pick a fight but it is too small for that yet, I'd think.