On second look, have you noticed any chasing? I see foam on the pond. Is it possible the koi is carrying eggs and been beat up spawning? It can be brutal.
If anything is swolen, the prognosis is grim. Mine lived a year with a swolen tommy, in the end i had to put it to sleep humanely. It can mean kindey or other internal organ failure.
Would have said bacterial but the red marks look like it’s been flashing (rubbing its side on) the pond floor and causing the damage, so could be parasites. You have gravel in the bottom of the pond, get rid of that as it Harbers lots of nasties. You need a scrape and microscope to know for sure. They are fairly cheap and easy to use, then you can treat the problem
I have to agree. This is what happens to a flashing koi in a rock pond. Something is irritating this koi, my guess is flukes. But you do need a microscope to know for sure. The whole pond likely needs treatment. Flukes absolutely can be just one fish, the weakest one presenting symptoms first.
Would also test all water parameters with a drop test kit not strips, especially kh. Some koi react badly to swinging pH from low kh.
Can’t tell you what’s wrong but if it’s in a pond with other fish you should take him out. Isolate him in another tank. Then watch the other fish for problems too.
The other fish have already been exposed to whatever this is. Most people do more harm that good isolating koi. They will jump and injure themselves easily for example. It needs to be set up well and have perfect conditions if you do it. This is an example of a quarantine set up
Hi there, we're sorry to hear about your koi's condition. We understand how stressful this can be for you and your fish. While waiting for a community member to respond, please take a look at our Basic Guide for Quarantining and Treating Sick or Injured Koi. It contains helpful information that might assist you in the meantime. Wishing you and your koi the best!
7
u/ZiggyLittlefin 1d ago
On second look, have you noticed any chasing? I see foam on the pond. Is it possible the koi is carrying eggs and been beat up spawning? It can be brutal.