It's difficult for me to be critical of the advice AMS offers in this thread. Keeping bumblebees in brackish water won't do them any harm provided the salinity is kept low (an SG of 1.005, as AMS suggests, is fine). AMS has experience of these fish, and that beats anything quoted from a book or web site.
The problem is that in the hobby dogma sometimes gets mistaken for reality. The other examples in the brackish water world are glassfish and wrestling halfbeaks. Both are routinely said to be brackish water fish. They're not. Conversely, many books say the figure-8 puffer is a freshwater fish, and it turns out it needs brackish water. Over the years I've learned from these discussions and the Brackish FAQ has been changed accordingly. Hence when I came to doing the Brackish book for TFH, I turned to breeders and scientists in the field to write specific chapters on things like gobies.
Not much written over the last couple of decades on bumblebee gobies is terribly accurate. Virtually every book and magazine has the wrong names applied to the wrong goby species, something Naomi Delventhal is working to make sure doesn't happen in the Brackish book. Likewise, I've tested out bumblebees in soft/acid water and found they will survive in it perfectly well for months, if not years, but they are not hardy, and I have lost more specimens this way than I'm comfortable with. On the other hand, Naomi has kept and bred them in purely fresh water with a neutral to high pH and hardness level.
There are also multiple species involved, and it is entirely possible that the species sold in the US, for example, are not the same as those sold in the UK. There may also be distinct populations within each species, with some occuring in brackish water, while others only in fresh. Hence, it's very difficult to give a categoric answer.
Bottom line, the idea these gobies
must have salty water is not true. That they may be
easier to keep in brackish water is something several experienced aquarists have observed, and for the less experienced aquarist it may be exactly the way to go.
Cheers,
Neale
PS. Statements like "Never try to acclimatise a non brackish fish to brackish water, freshwater fish have not adapted to take the high levels of salt in brackish water and it will kill them, the same goes for brackish fish into freshwater" are guidelines not laws. This is one of the things that fascinates me about brackish water fishes, that they are adaptable, and brackish water habitats are not one type of thing, but a whole spectrum.
For example, you can find Asian climbing perch, jewel cichlids, spiny eels, giant gouramis, common plecs, and x-ray tetras in brackish water in the wild. All will do better, however, in fresh water. So all the "brackish water fish" on that list can be adapted to fresh water conditions perfectly well.
What CFC is getting at is that "true" brackish water fish, in the sense of those particularly adapted to that habitat, like monos and scats, shouldn't be kept in freshwater aquaria. He is, of course, correct. The tricky bit is that recognising which are "true" brackish water fish is more difficult that some believe. Many species, such as glassfish, halfbeaks, and bumblebees, are supposedly brackish water fish, but either definitely aren't or are at least arguably not.
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