I would use potassium permanganate as a plant dip (10 mg/l for 10 min). I would never add it to the main tank. The high concentration of manganese released by this treatment may cause future problems for plants and invertebrates.Hmmmm I was just reading about potassium permanganate, and it is not effective if there is a high organic load. Has anyone with an el natural used this in their tank before with success?
Also, there are warnings not to mix it with sulphuric acid. Are there any small quantities of naturally occuring sulphuric acid in the soil of an el natural. I'm worried about the odd sulphur-smelling gas bubble that comes up from the soil from time to time.
That said, I think you are better off working with your snails rather than trying to kill them. I consider snails to be helpful in an NPT. They are great for cleaning plant leaves and recyclying fish wastes. If you feel you must get rid of them, you can bait them or get fish that will eat them.