Well I'm not sure what fish you're talking about, but we're all talking about the violet goby, Gobioides broussonnetii.
They do grow over 2 feet long. I've seen pictures and heard a first-hand account of two different specimens that have exceeded that size and are no more than 3 years old. Also, there are pictures of one in this forum that have made it to 16".
I don't know exactly what specific gravity the fish would do best in, but they move through a very wide range of salinities as part of their breeding. The salinity being too high seems kind of unlikely as a potential problem. Water quality is definitely the issue. Air breathing in this species is the fish's response.