If it is just one side, it's probably an injury. You could try plain USP Epsom salt at 1 tablespoon per 5 gallons of tank water. You can redose it one more time after a couple of days. It can help with the swelling. If this doesn't work, and remember fish are slow to heal, you could consider dosing in a QT Erythromycin. It works very well with eye infections.