Ideally, the Nile Puffer will need to move to the biggest tank you can get him (minimum 4x2x2) as they grow very fast. I know a guy on another forum who's Nile Puffer grew a foot in 12 months and is now 1.5' long!
Also, you might want to carefully consider tank mates. Nile Puffers are known to be particularly aggressive and will attack anything - including throwing themselves at the glass to get at people walking by.
Paul _MTS had some luck keeping a big mouth catfish with his for a while, but even though he's still pretty small and had a hard start, Cookie still managed to take a chunk out of the bigmouth when no-one was looking.
Best to keep them alone and yes, they are gorgeous, but do you really want to have a huge tank that could be full of lots of cool fish, but only has one cos he might kill anything you put in with him?