Do you have anywhere to put the snails for the duration of treatment? When had whitespot I moved the snails to my quarantine tank. But as nerites are so sensitive to medication, I do a massive water change and run carbon for several days after the whitespot has gone, changing it every day, then I run some polyfilter (the expensive stuff that absorbs medication) before putting the snails back. That may be a bit over the top, but I've lost nerites once before by putting them back too soon and assuming all the med had gone.