If you added another fish & it got the symptoms within hours, then something is in the tank. About the only infectious thing I know that would spread this fast is columnaris.
There still could be something in the water - toxins, etc.
I would agree that tearing down the tank would be safest. But if you are having luck treating its lone occupant, you might just keep going. I would do a large water change just in case there is a toxin problem.