Inflamed gill and possible dropsy?

[Marvel170]

Hey, so my girlfrind's betta has a inflamed gill, but one thing that is puzzling us about it is that his face started to turn gray and spread to his belly over the course of a couple of days. He is also being very lethargic, and only goes up to the top of the water for air and then returns to the bottom of the QT she has going for him.

His housing is 3.5g

Temperature is 75F

Water perameters N/A

Food Aqueon food pellets (blood worms as treats)

Is being treated with AQ salt
Thanks for all of your advice in advance :)

[ChoclateBetta]

I have to say probably something to do with water quality. Like ammonia burn.

[Marvel170]

How do you treat it? can it be treated with AQ salt? sorry for so many questions but I have one more, is the gray on his belly normal for that type of thing?