While i'm no expert on S/W puffers, i really think one of the main reasons your puff ain't eating is coz its not in the correct environment... i.e. a saltwater fish in freshwater, plus, with a fish that size, you should really be feeding it stuff like defrosted Mussels, Scallops, Market Prawns etc.. and not bloodworms... since i couldn't even begin to imagine how many cubes it would take fill up a puff of that size...
Anyway, i would check out the pufferpedia on the link that PufferPunk sent you to try and ID your puff... coz the sooner you know what it is and what conditions it needs the quicker you can help get it back to shape...
Lsat, but not least, how long are you planning on keeping your puff in the 3Ft X 18" tank for, coz that i'm sure is also adding to the stress factor a bit...