Hello and welcome. My name is ***** ***** I'm a biologist with a special interest in fish. I'm sorry to hear of this problem.
When a fish lies at the bottom of the pond, it only tells us that the fish is stressed, so that leaves a lot of possibilities open. You didn't mention anything as being already tried, so I'll give you some ideas.
A common cause of this is a lightning strike that caused an electrical surge in the water. When that happens, the fish nearest the pump is most likely to be affected. Larger koi are also more prone to this type of injury. There is no treatment, but fish do sometimes recover with time, especially if they are able to continue eating.
If the fish is female, have you considered the possibility that she may be unable to release some eggs from last season? When this happens, the only signs we may see are appetite loss and lying at the bottom on her side. If there are a great many eggs, the belly will swell, but if only a small number have been retained, we may not see that. The home remedy for this is to put the fish in a hospital area. 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. Then add Epsom salts to the water at a rate of 1/8 teaspoon per every 5 gallons of water. However, a better solution is to contact an aquatic vet. A hormone injection will often induce egg-laying. If the problem is severe, surgery may be necessary. This link will take you to a directory of fish vets:
A mild case of swim bladder disorder could also cause a fish to behave this way, but I suspect you have already considered this possibility. If that seems possible, it would be best if you can isolate the sick koi in a hospital tank. Once there, there are a couple of things that may help the ailing fish. If it will eat, give it some thawed out and peeled frozen peas. Peas can sometimes get things moving again, and ease pressure on the swim bladder. I would also add 1 tsp. 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. You may have to hold them right in front of the koi's mouth.
Parasites could also be to blame. It would be unusual, but not impossible, for only one fish to be affected. Other signs would include rubbing against the bottom or sides of the pond. A safe antiparasitic, such as praziquantel, would be used.
Temperatures too warm, insufficient oxygen, or poor water quality are other sources of stress. But here again, you'd probably see more than one koi with a problem.
Since there are no other symptoms, it is very difficult to determine exactly what is wrong. If one of the conditions I've mentioned seems possible, you could go ahead and try a treatment for that condition. If none of those conditions seem likely, I would contact a fish vet because the sooner the problem is diagnosed and treated, the better. If you have more questions, let me know. I hope your koi will reach a full recovery.
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