It does look like a wound from a fish biting it. What are the tank mates? At this point, I think that extra clean water will do more good than any meds you can add to the tank. Just keep a close eye on it for worsening. If it does get worse, then I would go with a broad spectrum antibiotic.
is your "my tanks" link correct? if this is in a 20g tank, you are overstocked to no end
the fish was probably bitten in that spot and got a secondary infection. do a PWC as suggested above and add some stress coat if you have any. if it doesn't get better in a couple days, start treating with anti-biotics.
those bala sharks will be over a foot each in a few years.