well there really is no green spotted puffer, there are green puffers and spotted puffers, use this site to identify yours, that will tell you about what level of brackish water it prefers
http://puffernet.tripod.com/confusion.html , as for foods, frozen and freezedried of all kind are good, they require a steady supply of snails to eat to grind down there teeth like mice, if they do not have hard things to eat like snails and crabs there tetth will grow so big they cannot eat, they are very agressive and should be kept alone, although i just purchased some red chromides which have settled in with mine fine, but the puffer is 1 1/2 inches and its in a 20L (30"), last time i tried to put something with it he bit through the bag and killed it,