http://www.applesnail.net is a great source of information. They say snails can only tolerate a very small amount of salt, if the concentration has been built up slowly. They also say...
Apple snails are very sensitive to certain chemicals and compounds. Unfortunately some of these chemicals are used to treat fish diseases as fungi and parasites. The basic principle in combating a fish disease is to use chemical compounds that kill the disease, while having no adverse effects on the fish, due to neurological/metabolistic differences between the organisms. However, snails have more in common with many parasites then with fish in the way they react to chemical substances. It's thus advised to isolate the snails in a separate tank during treatment of the fish unless you are absolutely sure that the product you use doesn't contain snail-toxic chemicals. And as many disease causing organisms do not survive outside the fish during a few days, together with repeated water changes in the isolation tank, the risk of reinfection through the snail reintroduction can be minimised. Exceptions to this rule are parasites that have a life cycle with snails and fish as intermediate hosts (mainly the case with wild-caught fish/snails). See also here.
A short list with chemicals that are/could be toxic to snails in therapeutic doses:
-Malachite Green (used to treat Ich or white spot, fungi and Velvet or Oodinium).
-Various organophosphorous pesticides like formaldehyde, metriphonate, trichlorphon (= dylox, masoten, metriphonate, neguvon, trichlorophon), dichlorvos and others used to treat infections with flukes, worms, crustaceans and lice.
-metaldehyde used as molluscicide.
-Various copper containing drugs to treat protozoa and fungus infections.
-Parricide D (Di-N-Butyl Tin Oxide) used to eliminate helminthes, acanthocephala, trematodes, cestoda and worms.
A list of fish phamaceutics of several manufacturers with the active ingredients is available on http://www.pubnix.net/~spond/product/medicine.html. Many of the preparation listed here could harm your snails!