Skin Ulcers?

Anyone,

I have an Amazon tank with three keyhole cichlids, two young angelfish, a
few cories, and a few small tetras.
Recently the largest keyhole developed a nasty looking ulcer on the back of
his head. He hides a lot, so he
could have had it for a few days. I did a 50% water change, cleaned out the
filter (Eheim), added some salt,
and turned up the temperature a bit, but he died within hours. I just
noticed a similar lesion near the tailfin of
one of the other keyholes. Any ideas?? Anything else I could do??

Pictures of the tank:
http://home.comcast.net/~zeno42/Amazon.htm

Pictures of the keyhole that died (the current ill one won't come out for
pictures):
http://home.comcast.net/~zeno42/DSC00029.JPG
http://home.comcast.net/~zeno42/DSC00030.JPG


Thank you very much,
Kevin Murphy