Balloon Belly Mollies

Question
I have 10 balloon belly mollies in a 30 gallon tank.  I have noticed that one of
the males is biting the other fish just under the tail fin.  Have you ever seen this?  
Why would it bite the other fish?

Answer
Sometimes peaceful fish become aggressive. Even though mollies are very peaceful fish, most of the time, they can be aggressive. It could be the number of fish in one tank or it could be he want to be the dominant fish and it could also be that he is just aggressive. Really the only way to fix this is to remove him, otherwise he will stress out all of the fish and they will likely get a disease or parasite. I suggest removing him. It not necessarily "normal" that he is picking on the fish but it is a possible thing to see. I have had neons, considered very very peaceful, and they have picked on each other. It really depends on the fish, the tank size weighted against the number of fish, or just the mood a fish is in. I hope this helps. Let me know if you need anything else.