>>769510First clean the tank / move the fish to another, then medicate the fish with methylene blue, the fish should be in isolation, also that bowl is not a proper tank for a goldfish like that so no wonder it got sick, the problem is that there is no water filter, air pump, temperature control…
If the fish has an internal bacterial infection antibiotics should be used instead.
But then again, on the image its just a generic goldfish, why would you put that much care for it? Don't even bother acquiring those, they're lame.