Hard to tell from the photos, but it looks as though it has a bacterial infection. That would explain the "pop" eye. Generally bacterial infections are secondary, often distracting and even hiding the real issue. Parasites are a good place to start(flukes or flatworms?). Has any livestock been added recently? Has the aquarium gone through any large changes resulting in stress for the fish, causing it to become vulnerable to infection? Best thing to do is pull the fish and quarantine. A good, general antibiotic never hurts. If you have any amoxicillin laying around it works really well, I believe the dosing is 250mg/20gal, but most fish stores have a few kinds in stock. Just make sure you keep up on the water changes, the antibiotics kill off even beneficial bacteria and tend to foul up the water a bit. Hope this helps.