Green spotted puffers are FW, but they do best in slight brackish. So, they are good for a FW setup.(in a species only tank) And only get to a max size of 3" to 3 1/2".
GSP are not freshwater. I wouldnt trust a tripod website, someone probably just did that page themselves with who knows what kind of source for info. A good source, however, like linked above, is the puffer forum. Look at the GSP info here: http://www.thepufferforum.com/forum/ug.php/v/PufferPedia/Brackish/T_Nigroviridis/
Most folks on TPF will say to start them out in brackish, and as they mature acclimate them to full marine.