It’s Popeye. If he’s in with other fish then move him to a QT tank. Use some of your filter media from your main tank so the QT can cycle quickly. Keep him in fresh water. Use Kanaplex and Furan-2 together as directed on packages. You will probably have to order them. You can give him Epsom salt (not aquarium salt) baths 3 times a day. This will pull the fluid out of the eye. Place one gallon of treated water (dechlorinated) in a bowl with 1 tsp of epsom salt. Mix well and make sure it is the same temp as his tank water. Place fish In for 2 minutes each time and then return him to the QT tank. Popeye is usually caused by poor water conditions so make sure tank is cleaned and frequent water changes. Good luck!
Do a 75% water change using de-chlorinated water and clean your gravel. Rinse the filter media in the water you removed from the tank. (Don't clean the filter if it is less than 6 weeks old). Continue to do 75% water changes every other day until it clears. I had this on a glowlight tetra and it took around 10 days to clear with no other treatment.