it could be something as simple as hormones.
There are many fish that emit a growth reduction enzyme when in the company of members of thier own species.
This is HIGHLY COMMON in schooling fishies.
OR it might just be the diff between sexes.
There is a wealth of info on the net about these guys becaus they are soo widely kept.
Look in to it.
I am sure that there is nothing to worry about though.