For some reason, Ich organisms don't cause white spots in Koi and most goldfish. I've NEVER seen white spot on Koi. At the same time, some of the hottest Ich infestations were on Koi. WITHOUT WHITE SPOTS.

If you see Koi or Goldfish turning red, getting slimy and depressed, it's usually a crashing pH but that's easy to prove.

Then if the pH is okay, you could break out your trusty microscope.

OR just start some salting.

Good article on salting over at savingsickfish.org

Or ask Sara. https://savingsickfish.org/ask-sara