The most commonly seen species of "dragon goby" is Gobioides broussonettii which is a brackish dwelling fish which requires salt to be added to the water up to a level of at least 1.005 SG. There is another species which is occasionally imported which can live in freshwater and goes by the name of Gobioides peruanus and is usually sold under the trade name of "violet goby", though both common names are interchanged and incorrectly used frequently.
To tell the two species apart you need to look at the flanks of the fish, on G.broussonettii (brackish fish) there will be dark vertical stripes along the whole of the fishes length (these may be faded if kept in FW or poor quallity water). On G.peruanus (freshwater fish) there will be no dark stripes. G.peruanus is also a much smaller fish which only attains a adult length of around 12 inches, G.broussonettii can grow to 20 inches plus.