Seem like you caught him in early stage. Raise temp to 86F-88F, and add salt (3g/L). It's best if you have a quarantine tank for him, drop the water level as low as possible, just barely cover its back (water level 1-2" above its back). It will help the arowana not using its strength try to keep it float. Good luck! I lost 4 arowana the past 3-4 months (2 are black, 1 leichardti)
It seems like black or leichardti are more sensitive than their cousin, silver or jardini.
Hope yours get's better before you have to put him down..
