If there are no large ulcers, then it is probably organ failure. The fishes ability to regulate the internal fluid levels is a two step system, the skin as a membrane protecting the fish from the low salinity external wanting to enter and dilute the relatively high internal salinity 0.9%. and the kidneys expelling any excess water from the system. When the skin is broken by a large ulcer the pond water has a means to overpower the kidneys by dilution of the 0.9% salinity. If there were a large ulcer, then antibiotics and high salt content of the water will be helpful. I have used salinity of up to 0.9% before and saved one of mine, but raising the salinity on a fish that doesn't have a large open ulcer would not be beneficial. Antibiotics do little for kidney failure and may exacerbate the problem as antibiotics seem to hammer the kidneys.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard