Does your koi have any external injuries? It can be scratches, open wounds, or you may not even see any visible markings at all.
- Open wounds:
It is definitely a bacterial issue. You need an anti-bacterial treatment. This can also be
indicative of a secondary infection from parasites, so you need to rule out the
possibility of parasites too.
- If you do not see any visible markings or blemishes on the body:
Most likely it is the gills. I recommend you put your Koi into a slumber with Koi Sleep
and examine their body from tip to tail, top to bottom. Ulcers and other marking may
not be visible at first glance. Open their gill cover, and take pictures of the inside. Even
if the Koi has already died, I highly recommend you take pictures of their gills,
especially if you can not find external ulcers. Those pictures will be helpful for your vet
or koi dealer.
Read more in our koi health checklist.