Ooh! That looks like some type of burn, especially for the flesh looking that raw, that large & that sudden, I’m going to say an injury. I personally have never seen a parasitic or fungal infection develop that quickly & that large overnight in less than 12 hours, those take a couple of days to start showing their ugly head, starting small & developing into a larger infection in the course of several days. Usually the loss of appetite is a good indicator for that type of infection but that looks more like an injury in my personal experience, I could be wrong. I would definitely QT & like everyone else suggested, lower salinity, decent flow, I have noticed they like swimming against current to sooth the injury, slightly higher temp & like Jay mentioned, a broad spectrum antibiotic to prevent further infection from the injury. The overnight appearance, going from 0 - 100 is what leads me to believe it’s some type of chemical burn, from what? That’s anybodies guess. Good look & speedy recovery to the Goldie. Again, I could be wrong but in my experience is what my eye is able to deduce.
