Bully fish etc..

Question
Tank is 10"x12"x18" (I think) about 40 litres?

There are 4 fish - Golden Fantail, Black moor fantail, calico fantail and a new 'comet' with standard tail (who joined the group about 2 weeks ago).

I caught the 'comet' chasing the Black Moor tonight, chewing his fins relentlessly. The Moor was quite pannicked and couldn't escape. His fins are now quite badly torn and chewed.

I have separated the comet from the other 3.
Why did the comet attack? What can I do?

Also, the Moor suffers from swim bladder or another boyancy problem quite often. Could his illness be something to do with his victimisation?

Answer
hello sammy! some fish are just mean and tend to attack the other fish. it ussually is because they seem they arent as bigs as the opther fish or as powerful so they take off their aggession on other fish in the tank.iam not really sure what you can do to stop it but keep them separated?  

  some fish loose their swim baldder and cant swim becuase other fish attack them.

hope this helps...feel free to ask any more questions.
megganna