by the sounds of things you have alot of problems with parsites i would of got a scrape done and treat from there as then you will know what you are fighting. i would never go to a store and put something in the pond [treatment] unless i knew what i was treating the big koi you speak about might have a swim bladder problem coupled with the above. a salt bath might help the fella.
fading colours says to me you will find costia and tric but without a scrape you cant be sure and will be chucking money away treating the unknown
