i actually grab my saltwater from the ocean , and mix with freshwater in a bucket , then test gravity /salinity with a hydrometer
only on water change though , when water evaporates the salt dosent, so just top up with freshwater ,
in the wild ,in estuaries ,mangroves etc.. there will usually be slight changes in salinity , with tides , rain ,etc..
i would test water the puffers are in and keep them in there , and gradually introduce salt , over a few weeks
marine salt NOT tonic salt
hope that helps , i am know expert , but this is what i would do ,and only my opinion