
We're sorry that you lost that cory.
Cory's can't be exposed to aquarium salt as they're scaleless fish. Malachite Green would be harmful to them as well.
A smell from the tank usually indicates some levels, particularly ammonia. Did you run the liquid tests within 24-48 hours of a water change?
I'm not sure of a safe med available there for the treatment of internal parasites, but aside from daily water changes of 25-40%, it's all I can think of to safely do without being 100% certain of the problem.
Do you have any pics of them to post? Can you get any of the following products: Fish Protector, Vita-Chem (or another liquid multivitamin) or Garlic Guard (or another garlic product)?