Sick rainbow fish -help needed!

Hi,
  I have a 4" long blue rainbow fish (Melanotaenia lacustris, I
believe) and although he is eating normally and behaves normally,
there is something very wrong; just behind his pelvic fins his skin
has burst open and what seems to be raw flesh or perhaps even organs
are hanging out!!
  Since I noticed this about three weeks ago I took him out of the
community tank and placed him in a hospital tank with 5 gallons, daily
water changes of 1 galon (20%), no filter, just aeration.  I have
treated the water with Furan-2 (nitrofurazone, furazolidone and
methylene blue) however no luck, I tried malachite green and no luck
either.  Adding kitchen salt (0.33 oz every two days) has not aided
either.
  So end result is three weeks later I don't see any improvement at
all.  I don't really understand if the problem is an ulcer, a tumor,
or something else.  None of the typical medicines has solved the
problem.
  I am putting up a photo of the poor guy at
http://www.polarpaper.com/T6ri/ for anyone who is interested in
checking this out.
  Any suggestions are most welcome!!  I really would like to save him,
he has been with me for four years now, I am afraid to put him back in
the community tank in case the affliction is contagious or other fish
might pick on him.
  Can't bear to see him in this state and am considering terminating
him instead of letting him live misserably in a hospital tank, very
possibly in pain.
  Please help!!


Jeremy