Treat this fish with Metronidazole. treat the tank and soak some food in metro desolved in a little water for about 30 mins and feed it to the fish. The eye looks a little swollen too. If you can't find Metro use API General Cure. Do a 50% water change and remove any carbon from the system if you use it. You can also add 1 tablespoon of salt per 5 gallons of water to the tan during treatment. If you do a water change during treatment only add enough salt back into the tank to replace what was removed by the water change. After treatment is complete keep the water as clean as possibile for the few weeks and we'll see if there is any secondary infection that has to be addressed.