Hi,
thid is Heiko Bleher and if I have understood correctly the Neons were freshly imported and you placed them into the community tank? If that is/was the case, then it is only very normal that thiose fishes began to die like flies.
A stressed, freshly imported fish, should NEVER be placed in a community tank, ONLy into a separate quarantaine tank where there is no other fish. And stay there under observation for at least 14, better 21 days. In a separate tank they do not have to fight the bacterias and parasites the other fishes in your community tank have and live with very well. The "newcombers" have no resistance against those (they need to first grow onto them, in your water, which in any case is always different from the water they originally come from).
No medication or treatment whatsoever will help now (nor before), and only the strongest of all of those will survive eventually. Only those which have a very strong immun system and can resist.
Please remember this once and for all times. NEVER place new fishes to your old(er) fishes, have new ones ALWAYS quarantained separately and you will never have this problem again (unless you get them sick and for that watch always you source).
From one who works with fishe over a half a century...
Heiko Bleher
www.aquapress-bleher.com