While I usually don't disagree with a more experienced fish keeper than myself. I gotta go with the low oxygen explanation. If that arowana was only stressed he would be laying down on the substrate or something, not sticking around near the bubbler.
Like previous members' advice, if I were you I would fill the tank up and get rid of the livebearers. They take up unnecessary oxygen in the tank and a suffocating predator won't be able to hunt live fish. Plus the livebearers will stress the arowana out. Also, I would do water changes and check your water temperature. Higher water temperature/high levels of ammonia can suffocate fish for various scientific reasons.