If you put a puffer in a tank that's too small for it, it will get stunted growth, and this will shorten its life expectancy. Puffers are notoriously bad for this, along with loaches.
Green Spotted Puffers need light brackish water as juvies, and full marine conditions once they're about 6 months old. F8s are light brackish fish (SG 1.004 to 1.008). There are a LOT of puffers that are purely freshwater, too.
Here's a short but accurate list of a few puffers:
http://www.pufferlist.com/
HTH
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