Looks like it's swimming around normally, just a little fat. How much do you feed it? Maybe just feed a little less for a few days and see if the swelling goes down.
1 thing you can do to reduce swelling (also for constipation--is it pooping?) is 1tbsp Epsom salt/5g. Best done in a QT. If it is bacterial, I'd post at www.thepufferforum.com, because we have the foremost puffer expert there, Kelly Jedicki (Puffer Queen) helping as a Mentor there.