If you are talkinv about the green spotted puffer, then yes they do best in saltwater in captivity. Keeping high bracking water is too expensive to keep up with weekly water changes. In saltwater, you have skimmers and chaeto to decrease the amount and frequencu of weekly water changes.
If you bought one from the lfs
in a freshwater tank, then drip acclimate them into saltwater. Research drip acclimation for these fish and you will find your answer. This has been done successfully by many hobbyist