I have a yellow lab mail that I noticed his tail fins were bitten into by someone yesterday....I will say that he has been aggressively trying to get with one of the females the past few days....would she do that to him if she is not interested?
It happens. If your YL starts getting pushed to a corner of the tank and swims with clamped fins, it's time to fix the problem. If it's just a nipped fin or two, no big deal. How do you know it's male, anyway?