Ive recently bought 7 neon tetra for my tank to go with the 4 older fish in order to get a better looking shoal effect.This has worked very well and they are happy and shoaling well, sometimes splitting into 2 groups with the younger fish following their older mentors.
Every morning I do a head count and untill recently I thought i had lost one although we never could find a body.Then my wife spotted the missing neon under where dave lives (pleco).It seems to be in good health but rarely comes out only to eat.
Is there a reason for this activity. The only thing i could think of was that it was maybe the only female and keeps getting set upon by the other 10.
AN IDEAS???

Neons can be touchy. If one stays away from thee others for a long time it's not a good sign. Who knows though? Good luck.