Salt's really not going to do anything here, IMO. And now, several people will come after me and say, "Add Salt!" I think salt is useful in SOME applications; however, these are far fewer than the ones it's actually prescribed for. From Lupin's sticky on salt: (
http://www.monsterfishkeepers.com/forums/showthread.php?t=226328 )
"How useful is salt used in aquariums and ponds?
From the number of aquarium hobbyists and pondkeepers who use salt one would have to assume that it is one of the best, if not the best, treatment to use for all kinds of known and unknown diseases. The reality is quite different.
It is quite possible that the unwarranted reliance upon salt treatments has resulted in more fish losses than the diseases themselves. This is because there is a common misunderstanding among aquarists and pondkeepers that salt is a good disinfectant, antibacterial, antifungal and/or antiprotozoal drug. At the concentrations commonly used in aquariums and ponds it is none of these things. As a reliable disease treatment and/or preventative salt is essentially useless."
Getting water quality back to optimal is going to be your best bet for "treating" this guy. After it's there, you'll need to keep it there with a good understanding of the nitrogen cycle. It's really that simple.