Epsom salt is magensium sulfate, not really "salt" as most people see it. It's used for ingrown toenails (yuck) and mixed with water to be taken as a laxative. Those are human uses, so I imagine it would also have a laxative effect on fish.
Rock salt is sodium chloride, just plain salt.
Table salt is iodized sodium chloride. Any type of salt is fine to use to treat fish, as long as it does not have yellow prussiate(spelling?!) of soda or caking agents.
As general care goes, I would not always keep my discus (or any other freshwater fish) salted unless they present symptoms to something that is susceptible to treatment with salt (a couple examples: hexamita with epsom salt, ich with plain/rock salt) and my water parameters are pristine (i.e. ammonia-0, nitrIte-0, nitrAte<40.)
If any of those water parameters are out of whack, I'd correct the water quality problem before taking steps forward with treatment. Bad water + treating illness puts an enormous amount of stress on the fish, which ultimately causes the fish to succumb to a completely treatable malady.
