Well, I don't have brackish and am speaking from limited knowledge, but..... if you see 1 tablespoon per 5 gallons, chances are it's not referring to a brackish tank. Some people use that amount with their freshwater fish, most often to help cure disease. A true brackish tank has 2 teaspoons salt per gallon for a salinity of 1.015.
Again, I don't have brackish, but I believe that's good information. I would wait on someone with hands on experience to chime in to confirm it, though.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~