tbh i'd medicate the entire main tank (ensuring you have no carbon in your filter) if all the neons are getting sick i'd guess the problem is in your tank not just in certain fish. what are your water stats in main tank? how often do you do water changes?
I have medicated the main tank and the neons get sick 1 after the other with something nobody has told me what it is. and this thing only affects my neons no other fish are sick and has been going on for a while with only neons. and not even gonna answer the other 2 questions as I know tthe stats are fine and I have a religious routine for them.tbh i'd medicate the entire main tank (ensuring you have no carbon in your filter) if all the neons are getting sick i'd guess the problem is in your tank not just in certain fish. what are your water stats in main tank? how often do you do water changes?