Calling all koi/pond keepers. Shubunkin disease?

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Apr 12, 2024
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United Kingdom
I’ve been keeping koi for a few years and have been highly fortunate never to have a casualty.

I recently did a 50% water change and hoovered the sludge from my pond—the usual clean-up after winter. I have replaced some filters by dipping them in the previous pond water.

The pond is 2.4m x 2.1m x 1.2m deep. I'm using a cup-7000 internal UV filter, which has never caused problems.

I have a total of 3 goldfish, 1 shunbunkin (starter fish), and 6 koi. All are roughly 10 inches in size apart from one that has had huge growth in comparison and is double their size.

I noticed yesterday during my routine observation that the shubunkin had some form of growth (I thought was cotton wool) on its gill. Decided it would be best to isolate and check but upon closer inspection, it appeared to be more like a blister with a yellow tinge.

I believe it may have injured itself at some point and an ulcer had formed but I've never seen an ulcer this colour before.

After netting aside using a rubber net, and holding him aside, he'd seemed to have given up on life. I proceeded to treat him with NTLABS ulcer and fin bacterial treatment.
But, he wasn't moving after two hours.

He was eating normal, swimming fine and all of that. No thrashing prior. I was just concerned incase whatever it was may infect the others. It seems to be isolated incident.

The parameters are all fine, too. First thing that I checked.

Please see photos.


Due to its coloration, it was hard to spot initially and that's why I believe I may have not seen it in time.

If anyone can help me understand what it may be, I'd be eternally grateful.


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Any chance of a picture of the fish in water?

Because of the depth of the pond and that it was at bottom of the pond majority of the time, I was unable to get a clear picture in the pond.

This is it in the isolation tub.

I found a video of when I recorded them eating and you can kinda see it here but I hadn’t noticed at this point.



I have removed that bit of fluff since taken. I have to manually skim the pond because the one I had installed kept malfunctioning. I do have a canopy over the top, though. So, it’s not a major concern
I can't tell what's what. It just looks like yellow colouration.
That was the initial problem I had when I first notice it but In person, I can see a protruding yellow blister filled with a fluid. I understand it's hard to see in the picture.

What doesn't make sense to me is that it gave up swimming after grabbing with the net. I've seen workers in fish stores catch fish more aggressively.

It may stay a mystery then. I'll just have to stay monitoring the others for now and hope it's just an isolated issue.

Thanks for trying to help though :)
You can try getting a video of it. Sometimes videos show things more clearly than still pictures. You can upload videos to YouTube, then copy and paste the link here. If you use a mobile phone to film the fish, hold the phone horizontally (landscape mode) so the footage fills the screen and doesn't leave black bars on either end.

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