Hey!! I have two spotted green puffers. Puffy and spotty (the kids named them). Originally purchased as fresh water I was told that they dont live very long that way. SO with a little research on the internet I was able to locate the SG for them. They like a slightly higher ph - like a chiclid (spelling?).
I brought my tank up to brackish, I believe it was like 1.018. I can look it up if you are interested. This took about 1 1/2 months so I wouldnt shock the little buggers.
I have had them for 2 years, and eventually brought them up to 1.024 which ended up being a good thing because their tank cracked and I had to add them to my reef tank. Boy that made me worry!! But they got along in there just fine, they stared at everything!!
I have to add this - funny story here....
My larger puffer likes to swim in circles around this plastic starfish one of the kids stuck to the wall of their tank (tank safe deco). Its his game, or something. Well in my reef tank I have a similar looking fish - choclate chip starfish. He started circling this critter and that poor puffer was clearly confused at how it started moving. He now stares at that plastic starfish int he tank and bumps it a little trying to get it to move. He hee hee.
BTW green spotted puffers go absolutely STUPID for live brine shrimp!! They love the freeze dry stuff too, avoid blood worms they HATE them. Ick, me too.
Good luck!
If you want any of the resource information I was able to find give me an email and I would love to chat about my silly little puffers!
fishinchick69@yahoo.com