What is “Flashing” in Koi or Pond Fish?
“Flashing” is basically just a fish that is “scratching” itself on the pond / tank bottom, or ornamentation. It USUALLY doesn’t mean anything, unless it’s “more and more common” or it’s happening at least hourly. There are lots of causes of flashing koi illuminated here (click).
Just some images (that may load slowly) of koi flashing: