I have a gold ghost koi that sounds quite similar. I got it at around 1½ inches back in May, and unfortunately it developed a bent spine and what appears to be a dislocated jaw. It got to the point where it became really, really thin, and despite everything it’s only grown to around 4 inches now.

I’ve moved it into my 800L pond where I can keep a closer eye on it, and thankfully it seems to be recovering well and putting a bit of weight back on. The bent spine and dislocated jaw are still there, though.

I’m planning to move it back into the main pond over winter so it’s not dealing with the colder temperatures in the smaller pond. It’s amazing how resilient these little fish can be!