I think it has gone past the cure by peas stage.
It sounds like bloat/drops/pinecone disease.
It isn't the problem, so much as a symptom.
doris
Here is some stuff I filched of the web-
Dropsy is a term used to describe the swelling of a fish's body. This causes the scales to stand away from the body like a pine cone in the forest, ... this is why it is also called pine cone disease. It is normally fatal
Dropsy is perhaps most common in ornamental carps of the Goldfish and Koi classes. If the fish has a sore, and the scales around the sore, or even on the whole side of the fish, stand up, does not necessarily mean Dropsy. These fish may respond well to injections of antibiotics. However if a fish does have fully developed Dropsy then survival is unlikely.
Fish care products and medication
Dr Johnson reports "the belly may become reddened shortly before death. Of the cases I have sent for necropsy, the diagnosis almost always comes back as a mixed infection by two bacteria: Aeromonas hydrophila and Mycobacteriosis.
The former bacteria (Aeromonas) enters the body through the intestine in the winter and springtimes, particularly; and also through ulcers, or the bite wounds of parasites like Gyrodactyliid flukes. The latter bacteria, (Mycobacteriosis) is not treatable with anything we can commercially utilize. The disease progresses despite your best efforts, and there is no known way to stop it in ornamental fish".
Dropsy is a serious, and usually lethal condition brought on by stress inducing factors like sudden water temperature changes and poor water quality.
Expert advice should be sought but recovery from true Dropsy is highly unlikely.
What to do if you suspect disease problems including dropsy
Test for Ammonia, Nitrite, and pH at a minimum, and preferably also test for Total Alkalinity and possibly Hardness.
Ammonias are lethal in their own right and worse at higher pH. If dead fish exist this will also create ammonia problem. pH can change overnight if the system loses its buffering capacity (this would show on a test of Total Alkalinity)
In the case of Ammonia accumulation:
If water quality is suspect begin a systematic daily changing of 20-40% of the total volume in the system. But beware of major temperature changes the system. A small addition of salt at .1% level will do no harm.
My first reaction was that it could be eggy but I'm not sure. Here is a few pictures - they aren't very good but it's the best I can do in 5 minutes!:
