Speaking as one who knows nothing about this myself, I've seen, on various sites, people anguish over how to tell the true freshwater puffers from the brackish, as there are a number of different kinds often misidentified .
Those who've wound up with up with the wrong kind for their conditions have found that while some will apparently live for a while, the wrong conditions will obviously shorten the lives of the unfortunates involved.
This may very well not be much of an issue for specifically dwarf puffers, but just in case, I Googled:
http
/www.wetwebmedia.com/FWsubwebindex/fwpuffers.htm
Lists a variety of small (and large) puffers both fresh and brackish/salt.
http/ezinearticles.com/?Puffers---Freshw...?&id=753198
'The reason most puffers die within the first months of captivity is that the local fish store fails tremendously to inform their customers about the requirements of their new fish. I have seen many puffers either mislabled or not labled at all in fresh water tanks, slowlly dying. The key is to know what species you are getting (even if your fish store does not tell you) and to know what kind of water they need.
'This is a quick guide to determine which puffers are from freshwater, brackish and marine. ...'
I've taken loads out and have them in a clean coffee jar next to the tank. Not sure what to do with them really. I dislike simply squishing them. Does anyone have a dwarf puffer who wants a nice snack? 
