Well are you gonna add a tbsp a day forever? What kinda salt were you gonna use? Marine or cichlid? I am not too familiar with puffers, have only kept a fahaka for a year or so. With him I used cichlid salt, 2tbsp per 10 gallons. Research the puffer and find out what kinda salinity it needs. If it needs marine salt you need to get a hydrometer (less than $10) Once the salt is added, you only need to add more salt when doing water changes as salt does not evaporate. If the water evaporates, the salinity will rise.