Nobody argues that violet gobies do better in freshwater than brackish. At best, you get a few people saying theirs lived a year in a freshwater aquarium without any problems until the fish jumped out or otherwise died "for no apparent reason". Violet gobies always do better in brackish water, end of story.
Brackish water is really anything between freshwater and normal salinity sea water. In practise, we tend to go for a specific gravity between 1.003 to 1.005 for the "low end", which is 18-25% the salinity of normal sea water. At the high end, we usually choose a specific gravity around 1.010, about 45% normal sea water salinity. If you download my Brack Calc application from my web site, you can explore the relationship between specific gravity, salinity and temperature.
As for the difference between marine salt mix and tonic salt, it's this: marine salt mix is only about 75% sodium chloride, whereas tonic salt is almost entirely sodium chloride. Marine salt mix also contains carbonate salts, chloride salts, and various other things that make sea water a completely different thing to just a solution of sodium chloride and water. It's sort of the difference between wine and grape juice -- yes, they're sort of the same thing, but not really!
Cheers, Neale
