Sorry, but When a fish is ill to the point of being moribund, and unable to swim, there is usually nothing that can done for it that will work fast enough to save it.
That said, I know people don’t like to give up on their fish. How is it breathing? If it is still breathing normally, and since you have it isolated, you could consider antibiotic baths in a bucket. Do you have any antibiotics handy?
Jay