Help! My puffer fish that I've had for 10monts is going to die!!!

[Toneloc66]

I have 7 other fish that are eating fine and acting fine. My puffer however stopped eating a few weeks ago. Now this morning I found him on the bottom, seems like he's really weak. I cant see any parasites or anything on him, only thing I see is his fins detiriorating (sp?). Doesnt look like nibbling from other fish. But he's breathing really hard and I dont know what to do. Im stumped because when I battled deseases before, I can see them on all the fish and treat all of them at once. But like I said, its only my puffer fish and he is the biggest one in the tank! Any suggestions? I know its pretty vague but from the looks of my puffer, he's only got a few days.