Zues, in the past I've kept brackish fish from my region and am in the process of establishing another brackish tank that I will eventually increase to marine levels. I would think that if you just took the process slow enough to and at a constant rate so taht the fish don't get stressed too soon you should be able to accomplish your task easlily over a matter of a couple weeks. It's my guess that those gobies probably were dumped from salt water ballasts into freshwater and had to acclimate or die when they were first introduced, so it's also my guess that they are quite hardy to begin with.
FYI, when I was a teenager I caught a blenny in the Chesapeake Bay and dumped him in my marine tank. I had forgotten to measure the bay water and acclimate the fish correctly and thought for sure that I'd kill that fish. It's breathing was labored for the first night but after that began feeding and lived for many years. Looking back it was hard on the fish but it survived. I would not put any fish through that again, but it does go to show you that some types of fish are quite hardy, especially those found in estuarine biotopes. In those types of environments, life is very harsh for year round inhabitants. Salinity can change dramatically especially during summer drought months after a large storms flood (like inches of rain or hurricane type storms) the watershed. Their choices are to migrate or tough it out, and for benthic creatures that's not much of a choice!!!!
Good luck!
