Hi Duncan & thanks for the reply.
Yes it is only this fish, & all koi are fine.
gill byopsy
That sounds a bit beyond me.
With regard to noticable gill problems, I really did not have the best light last night mainly because I can catch the fish easier that way.
I read with interest your explanation on fish becomming egg bound though, because I'm sure the carp has had a change of shape although my wife does not agree.
This carp is probably the oldest fish in the pond, 4-5 years old & has been at 20.5 for ages, obviously I could be barking up the wrong tree with the egg bound thing.
Would be a shame to loose it though even if it is just a common.
Sorry I just re read your post & missed your question, it's been off food for about a week, but I only really noticed the thick mucus layer yesterday, this fish has been off food befor & usually gets back into it. It's had patches of mucus befor that I have asked about on here but this just goes away normally.
Just to add I'm going to net it again shortly for a gill inspection although I'm not sure what to look for.
Mike