How long has this tank been set up?
What are the water parameters on the tank?
What is your usual tank maintenance routine?
Eye issues are usually a result of poor water quality or an injury. They can also turn into a bacterial infection, which can be deadly.
The best thing you can do right now is increase the frequency of your water changes and keep the water pristine. You might also add Melafix to aid in preventing a bacterial infection. Should the fish's condition worsen, you may need antibiotics.