Saltwater Aquariums: Aggressive Hippo Tang?

[DwarfLion]

I added a porcupine puffer into my aquarium and today is his second day in the aquarium and i noticed that the hippo tang nipped at him, i do not know if she is aggressive towards him or what. All other fish that were added after her died, and seeing her attack the puffer, is she the cause of my fish dying? is there anything i can do to stop her? she is my favorite fish, and i like the puffer. I have no other aquariums to separate them and i wish to not return or find a new home for either of them. please help