I agree with joanne that neons can be hard to keep. I think there are good and bad batches out there. Sometimes you get lucky and get a good batch. I have had my 7 neons for about 8 or 9 months. I have lost 1 so far in the 29g tank and it seemed to have NTD, but no other fish were affected. I never knew if it was. I was told that NTD wasn't all that common, so usually what you think is NTD actually isn't.
I also think they need a high number in a school. I think 6-8 should be the minimum for neons while people usually say 4 for other tetras. I think they need a good size tank also (i.e. atleast 20g). I had them in a 10g with a betta and they hid all the time. I moved them to the 29g and they are out alot. Even with a 3.5in opaline gourami who can be a bully at times. Its just from personal experience though. And it could be that I also have 7 glowlights, which they school with.
A planted tank also seems to help them survive. When mine were in the 10g, I started having a mini-cycle and registered 2ppm of ammonia. I never lost a neon. They seem to be very sensitive to water quality, but not mine...they were troopers. I think having plants and cover helps them feel more secure.
To me neons are like ottos...sometimes you get a good batch and sometimes you don't and most of them die.