It sounds like a Swimbladder disorder.
The swimbladder is the organ used to maintain buoyancy and stability in the water.
Generally a swimbladder disorder on its own is not a cause for great concern and often rectifies itself. However, if it is unable to take food or hangs on the bottom, infection can be caused resulting in death.
Fortunately it is not contagious and can be left in the pond.
I would take the fish out and place it in a hospital tank and heat the water slowly. It increases the Koi's immunity and will help with the healing process.
It is a good idea to increase the salt level to prevent a bacterial infection and give sinking food to ensure that he eats.
A vet can reinflate the bladder, but it is going to cost a lot of money and is only advisable with a very valuable fish.
Apart from this, there is not much that you can do.
It might also be caused by a bacterial infection or even in some cases an electric shock cause by a submersible pump, physical injury to fins or an overdose of medication, but I would rather diagnose the problem as a mild swimbladder disorder.
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