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.