Firefish with a regenerating fin

I have a firefish in a 55 gallon tank where the only other fish is a false percula clown. My firefish's top fin was beautiful and long when I bought it, then one morning while I was getting ready to feed them, I noticed the top half of it missing! Over the past couple of weeks it had grown back to its original state, but today it is short again. Is this a normal occurance? What might be causing it? The clown has never been aggressive or territorial that I have seen, so I don't thing it's him....

I'd appreciate any thoughts or ideas of what might be happening please!

 

Something in the tank is pruning the fin, probably the clown.

 

is it a normal thing that it regenerates?

 

Yes, very much like any cut on us.

 

that has happened to my firefish as well one time