Thank you for waiting. There are several potential causes of this symptom, but most common is a bacterial infection. A swim bladder disorder may also be involved. The swim bladder is an organ that enables the fish to properly orient itself in the water. When something goes wrong, the fish may float, swim upside down, or at an angle. We can treat for both.
It would be best if you can isolate the sick koi in a hospital tank. A large tote, such as those made by RubberMaid will work well. Use water from the pond and set up a filter or pump with an airstone. Once there, there are a couple of things that may help the ailing fish. If it will eat, give it a couple of thawed out and peeled frozen peas. Normally, you don't want to feed in cold weather, but the peas can sometimes get things moving again, and ease pressure on the swim bladder. I would also add one teaspoon of aquarium salt per gallon of water. Don't use table salt - it sometimes has toxic additives. Keep feeding nothing but the peas for a few days. To treat for potential infection, I would use Maracyn 2 or Melafix according to package directions. Both should be available in larger pet stores.
If you have more questions, just let me know. I hope your koi will be able to recover.
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