Neon Tetras

Question
I would like to add more neon tetras to my 40 gallon tank.  I currently have 5.  This tank has been established for 2 years.  In the past when I have added new neons, the currently established neons die.  I have experienced this enough times over the years that I quit adding them.  The tank has 5 pristella tetras, 4 corys and 4 otocinclus in it.  I have never experienced this phenomena with other species of tetras.  It's almost like the older die out to make way for the new.  I just want to get a school of 12 neons.  The tank is large enough and the conditions are established (7.0 ph 0 nitrates, 0 ammonnia, 0 nitrites, GH between 75-150 and KH between 120-180).  Thoughts on what is happening and how I can prevent it?

Answer
This is very peculiar.  I wonder if you are able to tell the new fish from the old so well as you are able to distinguish that it is the older ones that are dying.  Three things come to mind:  One, if you had a larger school than the established group you may be dealing with the fact that the older ones are now the minority and may not be as high up in the pecking order for food, causing stress, resulting in death. Two, some of the smaller fish may be carrying a disease that they may have become resistant to, but your fish may not be, and it is affecting them.  However, this is not likely since no other fish are affected.  Three, I have noticed that if you do put an even number of tetras in an aquarium, one fish up to several will die off to create an odd-numbered school that seems tailored for that tank, as if they are a little army of two by twos with one designated leader, it is a strange phenomena, but I can honestly say I have seen this last one happen in my own aquarium.