fish bully

I have a few fantails in a 120G tank and one smaller comet. The smaller
comet is
a "rescue" from a friend. Since then, I start to notice the long tails of my
fantails
seem to be bited by something. At first, I thought is the water, so I
increase the
schedule of my water change. Things seem to improve a bit until this weekend
I
finally have some time with my fish. Believe it or not, the new comer is a
bully! I saw him biting the long tails of my fantails whenever they are
within
attack range!

So, what should I do?

Fishbee