From what I am reading, it sounds like it's stressed and may have a bacterial infection PUFFER WITH PURPLE LINE AND WHITE BELLY This is what I would do (I dont know anything else Puffers so this may not help). 1st: Put in a quarantine tank 2nd: Try changing food 3rd: Wait see if that makes a difference 4th: give medicine Saltwater Puffer Fish Puffernet >>> puffer pathology Puffer Fish @ petfish.net I did find this: "It's very common to have a puffer die from blowing up in the tank, especially the Porcupine Puffer. When they blow up they may have trouble expelling the water and more important any air that they may have sucked in. This can kill them, especially if they've sucked in air. Puffering up is a very stressful thing and something that you should NOT try and make your Puffer do as it can lead to death. "