What is wrong with my Angelfishes gill?

One of my angelfishes (roughly 2 years old) has had this swollen gill for over a month now. He eats fine and acts fine. He does not gasp for air. All chemicals are great as well as the rest of my fish in my 50 gallon tank. It does not show symtoms of gill flukes. What could be wrong? A tumor? Any advice is greatly appreciated :-)