My expertise is more so marine fish, but I think I will bite (this is in response to the thread in the link, as well). Most of the commonly available livebearers live in either alkaline, hard freshwater or 'brackish' water--everyone already knows that. Some species, like monkey toes alluded to, can be found in full strength seawater and can live their for the remainder of their lives (i.e. P. latipinna). Most common species are (more commonly) found in estuarine conditions to begin with, having at least some salt content that usually fluctuates daily. So, in a sense, many people are correct in their 'arguments'. Some species do better in fresh-brackish ranging water, while others, like P. latipinna range from marine-fresh. NaCl wouldn't be absolutely necessary (unless there are 'true' brackish tankmates), but it really couldn't hurt, either, unless the tank has plants or houses actual freshwater fish that don't like too many dissolved solids. IME, most mollies do better with slight brackish conditions with the other params approaching seawater (pH and hardnesses). In some of the other threads, a link was made to a FAMA article, which I would have to disagree with somewhat There is simply too much variability to make such a broad statement (and some examples used are much less common varieties/species). Some of the species the author mentions are either incorrect or severely outdated, too. So, here's my opinion, for what it's worth--pretty much all of the common hobby varieties will do fine in all water conditions in regards to salt. Is there a preference between the water conditions for the broader ranging species? Probably not.
