Since i'm pretty sure a S/W puffer and F/W puffer's diet are going to be more or less the same...
You can also try feeding defrosted frozen shrimp, mussels on the half shell, home raised crayfish as well...
If non of those work, then the last trick is to get some fresh garlic, smash it and rub the garlic on whatever food your trying to get your puffer to eat. The idea is the smell of the garlic should entice your puffer to at least take a bite...
Good luck...