My fancy black telescope goldfish's left eye has swelled up over the past week by the day. He is housed in a 80-gallon tank set up about a year and a half ago. I last changed the water a few days ago, a usual weekly 1/5 water change. I use an AllPondSolutions carbon internal aquarium filter that cycles 450l/h and agitates the surface.
I messaged a friend, an experienced aquarium owner, with my problem. She recommended aquarium salt. I rushed to the pet shop on the first visible day of his illness (his eye swelled literally overnight) and bought API aquarium salt. I added as per instructions a rounded tablespoon for every 20l, dissolved first in a few mugs of aquarium water. It didn't help, and his eye kept swelling.
I messaged her again, and she said I was at the end of what she knew and I should post to fishkeeping forums for advice.
Although it could well be an eye injury (perhaps on some decor or by another tankmate if there is one), I would definitely treat for a bacterial infection - eSHa 2000 or Waterlife Myxazin are two good brands of treatment. [Incidentally, can you see anything moving in the eye? If so, it might be an eye fluke which would require a different type of treatment.] There's no guarantee the medication will work - it's possible it's a tumour/growth underneath the eye - but I think you should give this treatment a go anyway, to see if it helps.
For the medication to have any chance of working, you'll need to remove any carbon from the filter. If the carbon is a pad in itself, then that should be easy to remove. If it's contained within a sachet, then that may be more difficult. Edited to add: if this https://www.youtube.com/watch?v=bgpYuXrDBws&feature=emb_rel_end is the filter you have, then just remove the carbon tray while you're dosing medication, then, at the end of the treatment course, replace the carbon tray.