Is only one eye affected? If so, it may be an injury. If both eyes are swollen, it's probably a bacterial infection. How often do you do water changes, and what are the water parameters? Sometimes, popeye is related to a water quality issue.
For now, I would try an antibiotic, like Maracyn 1 or Maracyn 2, or TriSulfa. How is she doing with the Melafix? Labyrinth fishes like bettas and gouramis don't do well with Melafix. It seems to adversely affect the labyrinth organ. I don't have gouramis, and I think other members are split about 50-50 on whether Melafix is harmful to gouramis. But I know for sure that Melafix is bad for bettas. If you use it on your gourami now or in the future, just watch closely for signs of stress and do a large water change if necessary. But for now, I think your best bet would be to try an antibiotic.
How is she acting also without the large airstone? Labyrinth fish need to be able to breathe at the surface of the water, and if the airstone causes too much turbulence, it may prevent that. Maybe you could see how she does without it, or use a gang valve to reduce the airflow a bit.