Bangaii with swollen gills on one side?

[CookieRdReef]

I have a Bangaii Cardinalfish who has gills swollen on one side. I have had him for 1.5yrs and he is eating well. The gills have been swollen for about 2 weeks or so. Any idea what this is or what caused it? Thanks for the help!

 

[chaostactics]

Is it a single or do you have a pair? Is it still eating? If you have a pair maybe it's mouth brooding?

 

[CookieRdReef]

Thanks for responding. He/she is a single. Still eating great. Im looking into adding new fish but qant to make sure this fish is health first.