Fish with a gas bubble in its eye

[Raf]

WE have a batfish that has developed a gas bubble in its eye, The only issue we noticed with the aquarium it was in was lack of O2 due to not enough water agittation and a oily slick at the surface most likely caused by overfeeding. The other fish have no bubbles.

Any ideas of what has caused the bubble to form, what remedy can be done?

thx

Raf