Black Cardinal fish aggression

[tangs27]

Hello everyone I purchased 3 black cardinal fish about 4 days ago. All seemed fine until tonight, I was watching them when one of them went after the other and grabbed it on its mouth they were like that for a few seconds before it released the other one. I have never seen cardinal fish do this. Can anyone shed some light on what is going on? I thought cardinals were peaceful.

I tried looking them up but I can't find much about them. Thanks in advance.

 

[54seaweed]

sounds like you got two males and one female or just three males , and there fighting for dominance of the tank , females are the most docile if your looking to school them,
two females to one male is the optimal match up and will curve aggression

 

[tangs27]

Thanks for the response. Is there an easy way to sex them? I have no clue how to tell the sex of these fish.