Get bloodworm or live/frozen brine shrimp from LFS to start. They need snails to keep their beaks from overgrowing. Ask the LFS for some - they get them in on plants. My green spotted also likes squid. Don't feed them flake food- most likely they won't eat it.
Also any one looking for South America puffers PM me- I have a contact that is getting me some but I must take a min. of 10- I only want 5. I will be selling the other and can possibly get more than ten. let me know if you are interest and how many.