Hi there --
There are some misconceptions about acclimatising fish to different salinities. One is that the process needs to be done slowly. It doesn't, at least, not for truly euryhaline fish. Euryhaline fish -- e.g., monos, scats, mollies, shark catfish, soles -- have evolved to move between fresh and salt water freely. Many will follow the tide up and down estuaries, so will be go from fresh to salt and back to fresh twice a day. In other words, at worst they need 6 hours to go from fresh to salt water and another 6 to go in the other direction.
Many, such as mollies and scats, can be adapted much more quickly. I've certainly adapted black mollies from salt to fresh in two hours, and Schaefer, in the Aqualog book, says you can dump scats in freshwater straight into salt water. I wouldn't recommend this, but it's an interesting observation.
However, bumblebee gobies are not euryhaline fish and only tolerate a low salinity. They do not need a salinity above 1.005. In the wild they can live indefinitely in hard, alkaline freshwater (several species are freshwater-only, and the others occur primarily in freshwater). Even so, a good case can be made for saying they do better in brackish water in captivity.
Bacteria are different. They can adapt to a range of salinities, though freshwater and marine filter bacteria are probably different species (not sure anyone knows for certain). But they do not tolerate sudden changes. Even so, going from freshwater to 1.003 across a week or two should cause no harm to them. I've done this many times and never noticed a rise in nitrites/ammonium. From an ecological perspective, 1.003 is effectively freshwater, and the bacteria in the filter will be the freshwater kind.
So Fella is right in saying you don't need to do anything to the tank except add salty water gradually. I'd add a bucket of 1.003 water each time you do a water change. The bumblebee gobies will be fine in freshwater for weeks if not months, giving you plenty of time to change the salinity gradually. Really, the main reason people find bumblebees to be short lived is not salinity but feeding: they are unbelievably picky, to the point where they might only take a single brand of bloodworms! Frozen lobster eggs are greedily taken (by mine at least) and I'd heartily recommend you get some from the local tropical marine fish store. Live foods are useful, too.
On the other hand, penguinpimp1990's idea of a fishless cycle has its merits, and I'd recommend that approach if you were, for example, creating a strongly brackish tank (SG 1.010) to keep something like scats and monos in. For such a tank you would need a new strain of bacteria for the filter, most likely different from those in freshwater.
Cheers,
Neale
