Crepe: I've had all four neons for two days today. None of these four died, but I did lose all three neons on day one of the tank to New Tank Syndrome (when I was even more ignorant than I am now). The two sets of neons never met in my tank.
My tank has been running for 10 days (I use SeaChem Stability to fast cycle). Everything in the tank was bought at PetSmart. I have no clue how old the neons are. I feed them flakes 2x daily and the ADF
gets Frog Bites. This neon doesn't really eat - at least not that I've seen (certainly not during feeding time).
I don't have any recent water test data. I don't think a picture would help as he LOOKS perfectly normal - behavior is the issue.
OhNeil1969: The tank also houses three male guppies, ~15 guppy fry (four days old), one molly, and one ADF
Fishguy1: Yes, a quarantine tank would be great in this case. Unfortunately, it was a fight with my wife just to get this one tank. I suspect a QT may happen eventually, but it could be a while. Therefore, my only options are to leave him in my tank, or return/replace him.