RadleyMiller;674094; said:
????????
Brackish is 1.001-1.019 IMO
The bacteria will not be different to the fish, ammonia is ammonia. Brackish fish can handle changes in salinity so it isn't that big of a deal to change salinity when you bring them home. Just drip acclimate them like you do with salt. Not all puffers are brackish and it may be a freshwater puffer you bought. If it is, then it won't live long in brackish.
I agree, that brackish water is anything from 1.001 up to fill seawater. The difference is the bacteria that break down the ammonia and nitrite. up to 1.005 the bacteria is the same as it is in fresh water, above that, the bacteria required is different. This is why you can't clone a freshwater aquarium to a saltwater aquarium, the bacteria required is different, and the freshwater bacteria can't survive in water above 1.005 and vice versa.
The majority of puffers traded are in fact freshwater, but they are all to some extent evolved from sea water so can handle salt reasonably well, however, you want to give it the right conditions and ID'ing it is the first step in doing that.
Can you describe the fish in any more detail? active, dormant? Dark coloured, ligt coloured? eyes on top of head? on the side? etc