Can't help with the diagnosis-I have some ideas but am not an expert in any fashion and wouldn't want to send you down the wrong path.
However, when one of my goldfish had a parasite, my "Pond Guy" (he does the heavy lifting so to speak as I am unable) set up a "hospital" using a 20 gallon plastic tote/bin (like you use for storing stuff) adding 75% pond water, 25% fresh water (with dechlorinator) and a small air stone. This way I could hopefully stop the spread of the disease, and treat 15 gallons of water with "medicine" instead of 2,000 gallons. I did 25% water changes every other day.
BTW, my fish was NOT happy with this arrangement-started swimming full force head first into the walls of the bin.
And a confession: After treating for 2 weeks I made a TERRIBLE mistake-there was an algae bloom practically overnight in both the hospital and the pond. I treated both with an algae control product, using the amount indicated and the next morning my little patient was gone.
Hopefully you will have better luck, and remember that I am by no means an expert but have done a TON of research and my pond guy is such a great resource for educating me about my pond.
