I agree, that is fin rot. My friend's silver arowana had that too, but more severe. It starts out small but eventually it will eats away at the fins dramatically. And it can spread to the other fins too. From your picture and video, the situation looks like it has gotten worse. If you have a smaller tank to treat the aro by itself that would be best. If you can't then you should lower the water level to at least 50% of what it is now. What I did for my friend was use salt ( 1 Tablespoon per 5 gallons), stress coat and melafix - follow the directions. We treated it in a 75 gallon with the water half way full. It took about two and a half week before there was any improvement. I think that this is the best choice for your fish now. Don't wait too long because the fins may not grow back to the same length and size.