Hello again. Because I don't want you to have to wait any longer, I'm going to go ahead and answer as best I can without your response to the questions I asked.
There is more than one possibility for what is going on. My first suspicion for what you have described is what is called 'pop eye.' This is not one particular disease, but a symptom that can be caused by several different things. Sometimes it is the result of an injury.Often, it is bacterial. In that case, it is recommended to treat with myxazin. You would need to put the fish in a hospital tank because we don't want to treat the whole pond. Here is a source for the medication:
Pop eye can also be the result of a vitamin A deficiency, but if you're feeding a good commercial food, that is unlikely. Kidney failure can cause fluid to accumulate behind the eyes. Excessive nitrogen gas in the water can be another possibility. This is most likely in ponds where there is a lot of agitation of the water with power jets. Koi do also get actual tumors, which can put pressure on the eye from behind. A less common cause of these problems is a parasite infestation.
As for what you should do, the first step is usually to treat for bacterial infection.. Adding aquarium salt to the water can help in some cases. Add one teaspoon for every gallon of water in the hospital tank. If you want, you can also treat with an antiparasitic, such as Praziquantel, on the outside chance that parasites are to blame. If there is an organ failure, nothing you do will help. If there is an actual tumor, an aquatic vet can remove it, if you want to go that far to attempt treatment. Many fish live comfortably for quite some time with these eye conditions. If yours is eating and active, that may be what will happen. Sometimes an eye actually dries up and dies. A fish can adjust to that. If the eye symptoms are the result of an internal organ failure, other symptoms will eventually appear.
If you have more questions, let me know by clicking on REPLY. I hope this condition will respond to some of the measures above.
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