Use one tea spoon of Epsom salt in a five gallon bucket of tank water bath the fish for 30 mins. If you have or can get methylene blue you could add one teaspoon to the bath. This caused by fluid uild up behind the eye the epsome salt will draw the fluid out and release the pressure.
This symptom is usually the result of a bacterial infection, caused by deteriorating water quality. I would test your water, ammonia, nitrite & nitrate hardness and tds if possible. Do a very good substrate vacuuming, clean your filters this is important. Stepping up water changes in the future will help too.
Back to the fish, most eye infections are gram positive caused by streptococcus or some other related bacteria. The first medication (other than the above mentioned bath) I would use is erythromycin. After the initial bath above and tank maintenance, do a twice daily 30 min bath in a bucket of tank water using the erythromycin dosed as per package instructions for 10 gallons of water. Do this for 5 days. If you see no improvement in 3 days let us know and we can switch meds, this, however will work most of the time. It is important to keep the main tank as clean as possible during treatment. Hth.
