awwes... poor guy. I dont think you should use AQ salt, and I dont think a fly caused this, its actually common with opaque bettas.
Heyyy you might wanna check this out :/ I also have an opaque male betta who is sick with the same thing! He is 10 months old and a few months ago he started out with those SAME red streaks on the tips of his fins! I first thought this was fin rot but later he was diagnosed with septicemia ( also known as "red pest disease") , an internal bacterial disease with the association of rotting denigrating fins commonly confused with fin-rot. So if you have been treating him for the fungal infection fin rot and nothing is working he might have a minor septicemia case. :( I would just keep up with daily water changes for now until a proper diagnose.
I attracted a picture for you so you can compare. Mine was a HM 2" long caudal fins, but he is starting to loose his caudal fin. I really hope your fish doesn't have this, as I have been battling with it for months but keep it in mind if the red streaks progress. :( Occasionally, when the bleeding effect goes away on a certain spot, it also turns brown. ( old blood?) Izume still has it pretty bad right now but he isnt lethargic-in fact, very active and he responds to food immediately.
**I read up that the gene that causes opaque bettas to be this color also causes promotes certain organ failure and disease.
♥♥♥♥ Best of luck to your fishie! ♥♥♥♥