When I had my pond fish in QT for fungus, it almost died, and someone suggested as a last resort, putting another fish in with it to keep it company, as they are social fish. The difference in just an hour was amazing! It started eating, swimming around again, etc. BUT, it had already been medicated so it couldn't pass on the disease. If yours has something funky, it might be risky to expose another fish to whatever it has. If no one responds, you might try a private message to Lupin, Inka40, or Flaringshutter. They have all had very good, reliable info for fishy illnesses and treatment.
In the meantime, the cleanest water possible is the best I can offer :/
Jen
PS. Another thing they recommended to get a fish to eat...squeeze some garlic juice on the food, they like it. I also got some live brine shrimp and frozen bloodworms to tempt mine.
Jen