i've only had mine a short while, but the profile in dr eberts book on puffers says that he has kept with some success in fw, but its lifespan and size wre greatly reduced compared to those hes kept in brackish or marine water. General rule seems to be light brackish as juvenile, increasing s.g. with age, and if you want you can go to full marine with an adult specimen.
keeping in freshwater seems to make them a lot more prone to fungal infections and other problems too - i bought mine in freshwater and immediately increased to light brackish and have had no problems at all; within a week most of the fish from the same batch remaining in freshwater in the shop had ich, and they lost a lot of them