Yikes! This is what happens when a thread gets hijacked. However, if you sift through it with a fine-tooth comb, you should be able to gleen the following conclusions on the two issues raised here:
1) Whether you need to add salt to your Datnioides tank depends on the species you are keeping. Of the five Datnioides species, widebars (D. pulcher), Indos (D. microlepis), and thinbars (D. undecimradiatus) are generally considered to be FW species, i.e., not needing salt. Silver dats (D. quadrifasciatus) and New Guinea dats (D. campbelli) are generally considered FW and BW species that benefit from some salt. Of these two species, I would say that NGTs are easier to raise in a completely FW environment.
2) Regarding downset21's unrelated query about aquarium salt, guppy's comments are right on. Aquarium salt typically is only sodium chloride, the primary but not only salt in SW. Marine salt mixes contains sodium chloride, potassium chloride, and other salts that occur naturally in SW. Thus, use aquarium salt for treatment of illness. Use marine salt mixes for creating a BW or SW environment.