I'd say your fish is suffering the after effects of sensory deprivation. Lock yourself in a room without light for 2 weeks and see what happens.
You have to remember that Arowanas are sight hunters active in the daylight, not at night.
You say with the lights off the Aro "acts normal", but I don't see where you address whether or not this fish is eating. My Arowanas don't eat in the dark. So is yours eating or not?
As a possible 1st step towards bringing your fish back to some modicum of normalcy, try some very low moonlighting effect. Either LED or even a short string of blue Xmas lights.
But BD is abolutely correct in recommending isolation, because those Motoros will gnaw on him as soon as he hits the bottom of the tank. so far it's just been the tail and fins but one of these times it's going to be more and you'll awake to a half eaten Aro.