i just buy sea salt from bulk store, i put one full cup in the fish tank, is completely fine, i do this all the time, and salt gets evaporated all over my tank (out side the tank), i don't do it for brackish, i do it just to soften, and kill ich. i once put 8 cup of salt, in one week of time, water did not change, no new water added, fish still didn't die....
the cup i'm saying is (MEASURING CUP) 1 FULL CUP