You sure it's not the silver dollars doing it to the arowanna and eachother? The only SDs I ever had were jerks like that. If the chunks are really big you may end up with permanently amputated or shortened areas of the fins....take a look at the average Large silver dollar, a lot of them have this kind of damage. Barring infection these areas will heal over on their own.
Assuming your tank is large enough you can always try putting a few more hiding spots or "view obstructing" items like big plants in the tank so there are fewer quarrels over territory.
Also make sure all of your parameters are OK as this can contribute to slow healing.
Good luck!