black marks on outside of gills

Question
I have a goldfish in my outside pond and it has got (new) black marks on the outside of its gills - quite even on either side - is this a problem or just marking on the fish?  I have had my water quality tested twice recently and its been fine and also treated the pond for parasites & bacteria as one of my koi has been listless and not eating.  The fish appears healthy but I obviously do not want to risk the health of my others

Answer
Hey Lucinda,

Black spot disease is excruciatingly rare in goldfish, but I've seen it. It's a protozoan, similar to ich in how it spreads, and how it's treated. However, it's very, very rarely symmetrical, and even more rare to be just on the gills.

Another possibility is mites, flukes, worms, etc. Parasites would have been eradicated when you treated for bacteria and parasites perviously though. It seems to me that this could just possibly be coloration changes. As goldfish (and all fish) age, they do tend to change coloration slightly. I notice in my guppies especially, that a bright purple tail darkens into black, fades to red, and can end up blue, green or even yellow 3 years after i purchase it. Keep close eyes on the goldfish, and if the spots seem to become raised like ich, pitted like hole in the head, or cause sores, swelling, or lack of desire to eat, pick up medication right away. If it raises, causes sores, or slows his eating, get black spot disease treatments. If it pits and infects, get hole in the head, or anti bacterials.

Hope this helps!