I had the exact same thing about two years back. Turned out to be a predator,
my oversized, vicious, killer frog. I tried all sorts of meds, water changes,
water testing, etc. Frogster stood watch throughout. Koi and Shubunkin were
"nicked" and showing the same symptoms as you describe.
One day, Frogster chose to disappear and the water inhabitants almost
immediately returned to normalcy.
Good luck!