Part of fin bitten off: Help! What do I Do

[GallonsOfFun]

So I got my two new Green Spotted Puffer's today (yay! - a post will come soon thanking you all), but I noticed one has half of a fin missing, with the top part of the fin being red. This leads me to believe it's been bitten off.

As you can see in the photo (closest puffer to camera), it's the fin just before the tail and you can see the red. What should I do? Please list what I need to do (if anything). Will the puffer be ok and will it grow back?

I really appreciate your help.

 

[GallonsOfFun]

Here is the photo.

 

[SpaceFish42]

I think the best thing to do now (if you haven't already) is separate it from the other fish so it doesn't happen again.

 

[GallonsOfFun]

I just got it today, it was like that at the store - it wasn't caused by the other puffer in the tank. If anything, this one is the more aggressive one.

Can anyone provide treatment info? I've been told to add Melafix and Pimafix to the tank. Do you guys agree?