I read something about them a whlile back about killing each other off one by one. I have had a pair in mine for the last two years that I would love to catch and replace them with a little more appeal since I only have 5 fish in mine.
A while back I did by another pair for a total of four, but two died shortly there after......
I have heard the odd numbered thing before, but I am pretty sure that even though they are schooling fish, they can't count......(get it, schooling!!!)
Its probably a herem size to gallon/territory thing and when they peck each other off into a small enough group for the area, they are fine.....hence my only keeping a pair intead of 2 pair.