My feelings about the possible cause would be electrocution, either by pumps or other appliances leaking into the pond, or lightning, or it could be stroke. I don't think it has anything to do with parasites, swim bladder, bacterial or viral infections which can be fixed. The fish that I have seen that swim like that have a broken back, making the back non-responsive making the front do all the work, and the fact that it is swimming in a circle it is acting like one side is worse than the other. I don't like to recommend putting a fish down until all remedies have been put to the test, but the only thing that might help this fish is time, and I think it will work itself to death if given time. Sorry, but I think the best treatment for this fish is heavy sedation with clove oil and an hour or so , which will result in death.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard