idk about this.
i keep both fresh and salt aquariums (and not exactly cassualy mind you), and i have gone so far as to adapt my share of saltwater caught fish to freshwater, although none that are exclusively found in marine environments. ive kept silver mullet, urobatis halleri (california round ray), american eel, and juvenile "spot" in freshwater that i caught in marine environments.
imo this magic water is just packaged hard water. specific gravity doesnt just apply to salt, it applies to any dissolved electrolytes. i dont want to get technical or chemical at the moment, im nursing a terrible sunburn....
marine and freshwater fish are not entirly different anatomically. the main difference is that saltwater fish drink and urinate to keep their salt content stable wheras freshwater fish (from what i understand) do not. if this product somehow adds just enough electrolytes to replenish the salt in the marine fish while at the same time not being detrimental to freshwater species i see no reason why it wouldnt work. however, im no biologist....
idk, im sure wes will come charm in soon enough on this thread. this is right up his alley.....