Young arowana can be skittish. It will get less frightened the older it gets. Each arowana has different personality. Some are braver and others more frightened. My bigger arowana was raised from last year. When it's first arrived, it darts around the aquarium when someone approach it. Give it some time and it will get less skittish the older it gets. It may take weeks or sometimes months to get over its frightened behavior.
Covering aquarium with cloak can help your fish to adapt better. Location of aquarium also plays a role in new fish adapting to tank environment. Bad location includes placing tank near window (light reflection from window will make it harder for new arowana to adapt) and placing tank too low.