The tail of my goldfish has become thin (see photo). How to treat it?

Are there other kinds of fish in your aquarium? If it is bitten by another fish. If other fish bite, then you can only make one choice, either raise goldfish (golden crucian carp) or raise other fish.

Your fish is terminally ill and cannot be cured. If you want a radical cure, you can try this; If other fish bite; Separate it from other fish. Put some furacilin, rest for a while, and see if you can recover.

If you get rotten tail disease, you have to keep the sick fish alone, and then apply malachite green 1% water solution to the affected area, once a day, for 3 ~ 5 days; Then spray the whole pool with 1 ~ 2mg/L of furacilin or furazolidone. ② The whole pond is sprayed with rivanol 0.8 ~ 1.5 mg/L, which is suitable for rare varieties.

But I think your fish was bitten by another fish:),