You can try Melafix and salt. Melafix is good for healing of fins and good for dealing with bacterial infections. Follow the instructions/directions on the label. Maintain excellent aeration.
Make sure you do not have anything that absorbs chemicals -- no carbon, no zeolite, etc. If you dare, refrain from feeding your arowana for one week and see any improvement.
melefix is actually garbage. There is no need for it. I've tried melefix tons of times and it actually does more harm than good.
Clean water and aquarium salt is the key.