Have you tested your water? If you have, can you post the results? I believe higher ammonia can cause “clamped” fins on Koi.
Here is some info:
Fins Clamped to Body
Fins Clamped
This symptom is most recognized in the pectoral fins (front fins just behind gills) and the dorsal fin (top fin on their back). The fish will clamp their fins so tightly against their bodies it appears that the fins are not even there. Many times this symptom goes hand in hand with two other symptoms listed elsewhere. They are isolation and sitting on bottom. Most often it is only one or two fish that exhibit this behavior and not the entire population. If the entire population shows this then you have very serious problems and most likely multiple problems of the most likely culprits listed. Again, this can be a sign of poor water quality, bacterial infection, parasites, or physical injury.
Try to capture any fish showing this symptom and inspect them for any physical injury. If you find none then the issue is most likely to be bacterial and/or parasitic.
Check all water parameters: ammonia, nitrite, pH, kH and in addition check the nitrAte with the occurrence of this particular symptom. The ammonia and nitrite should be ZERO, and the nitrate should be below 50 ppm, but as high as 150 should be ok as not causing this symptom. If all water parameters are good, then it is most likely to be parasites and/or bacterial.
In this situation I would start with the salt to .3% (point three percent) but done in .1% (point one percent) daily increments. Give the fish at least a week in the .3% and see if the symptoms stop. If the symptoms don't subside after a week, I would move to a more serious set of treatments listed in the Treatment section of this site. If the symptoms don't improve after the salt for a week, I would treat for protozoan as well as flukes. One other possibility is fish lice or anchor worms (crustacean parasites). When in the adult stage these can be seen by the naked eye.
