You need a hydrometer to measure salt (salinity, SG). Brackish water is generally about 1.008 to 1.012 on the hydrometer. This salt needs to be maintained by frequent checking and adding freshwater to make up for evaporation. Salt doesn't evaporate with the water, so the SG goes up. Freshwater levels this down.
I'm wondering if the store you got the puffers from would know if it is the Tetraodon schoutedeni (which are freshwater and only get 4" from Africa) or the Tetraodon nigroviridis (this is the brackish species and gets 6" from SE Asia). These two species are nearly identical in appearance. I've no clue what the tell tale difference between the two would be for positive ID. Do an image Google search and you'll see what I mean. It wouldn't surprise me if one was sold as the other from time to time due to mistaken identity.
You could just not put salt in there as brackish, but dose it like one would dose a pond. One tablespoon per five gallons. 7.4 would be an ideal pH value. Puffers certainly aren't community, so might as well just keep them as pets instead of a 'decoration'. They do love sand for substrate with plants along the borders of the tank. Flower pots make excellent hide out 'caves'. Leave the mid section of the tank open for free swimming. Plants suitable in sand bottomed brackish tanks are Java fern, Java moss, mondo grass, and any of the several species of anubias. Anacharis might do OK and hornwart. These two particular plants can be allowed to float to provide surface cover as well as rooted into the sand.