OK. The Utsuri is now in my 4-gallon isolation tank with a dose of Tetracycline. I re-built my isolation filter with new filtration pads and my ammonium neutralizing particulate. She is still lying on her side at the bottom of the tank with a upward curve, but every so often she will swim in a circle upside-down and the drift to the bottom of the tank again.
What the pet store recommended was using the same water that was in the original tank, which I filtered (
ambug666 helped) with a filtration pad, to eliminate some of the algae that I had just scraped off the sides of the tank
1. This means that the original tank is now 4 gallons down. So I have to take care of the main tank, too.
The Ogon is swimming around in the remaining water, sort-of despondent looking, and
ambug666 remarked that he might be missing the Utsuri. I rather more cynically replied that he was probably unhappy with the tank water, which had a lot of scraped algae floating around. He still looks healthy, so we'll keep an eye on him and see what happens.
1 Lest you fear for the health of Paris, there is still plenty of algae on the rocks for him to eat. OK, the snail is now a "he", I guess.