Aggressive sailfin mollies.

Limeylemon

Hi, I have currently in my tank, 2 sailfin mollys, 2 regular mollies, 2 Swordtail mollys and 3 balloon mollies, with tetras and other fish.
Ive noticed recently my sailfins are getting rather aggressive with themselves, the normal and the balloon mollies. chasing them around constantly , nipping at their underbellys like their trying to mate!
I have no idea how to sex them to figure out if it is mating behaviours or if they are just being ratty.
Any ideas on what to do?

Beaches

This older thread shows the difference between the sexes. It is the same in all Livebearers, it is relatively easy to tell by the shape of the anal fin, the females is faned shaped and the males is spiked shaped (called the gonopodium, which is the males sex organ)....how to tell male or female

It could be a bit of both LOL, I have found that Mollies tend to be on the aggressive side, but the behaviour your describe is also normal for all livebearer species. As has been mentioned in many threads, the males will constantly harass the females (even to the point of exhaustion and sometimes death for the female) in an effort to mate, there ins't much you can do about it if you keep males/females together, it is instinctive, which is why it is best if keeping both sexes, to only have one male and the rest females. Also providing as many hiding places/bushy plants also helps. If you find out you have a lot more males than females, it may pose a problem, as males can show aggression towards the other males and the female will get harassed/stressed.