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I agree with maxwell. Also, other signs are excess stress shown in poor coloration/death. Aggression, bullying, excessive chasing, and other behaviroal issues also can occur. Of course, these things can have other roots as well, but they can pop up from overstocking. Also, if you are overstocked, the fish do not tend ot grow as fast IMO.
I agree with siva. To my knowledge, they are schooling fish, so they would probably do best in a little group. Getting 2 more sounds fine. Put up more pictures if you have them!