My Cichlids and their behaivor

Question
I have 3 cichlids in one tank:  1 Nimbochromis Venustus, 1 Green Terror, and 1 Demasoni.  All juvenilles.  How would I go about determining the sex of my Green Terror?  
The Nimbo. Venustus is flashing fins and gliding around the Green Terror who seems to be annoyed by these actions.  What is the meaning?  Fighting or mating?  


What is the purpose of sudden flashes of rubbing against smooth surfaces mean?  Also, if this is a problem how do I fix it?  

Answer
Hi Pierre,
  You can't sex juvenile green terrors in my opinion. It is even very hard to sex adult green terrors.  
  The behaviour you are seeing are most likely typical signs of cichlid aggression.  

   Flashing against rocks can be a sign of external parasites so keep an eye on them and if I were you, I would have some ick medicine handy but don't use it unless you see ick for sure.

-- Ron
  rcoleman@cichlidresearch.com
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