Ingrid help, anybody else can jump right on in with suggestions.

I noticed some of my GF have black in their gills (inside). Is it something
to be concerned about. I have tested the water and all seems fine. They look
fine none are wacking out. None of my Koi have this. I never noticed this on
them before last week. What made me notice was one of my ugly black GF that
I thought was gone showed up a pretty red gold then I noticed the black
inside the gills thought it was his coloring then I noticed some of the
others had the same thing. I have read about gill damage, is this it? Joann