Brackish water fish will often migrate into freshwater. They often do very well in coastal freshwater habitats because they can tolerate occasional changes in salinity caused by, for example, freak weather that pushes salty water farther inland than usual. So finding a brackish water fish in a freshwater river or canal is really no big deal.
The difference between a brackish water fish and a true freshwater fish is that brackish water fish cannot normally complete their life cycle in freshwater conditions. At the very least, they can't breed. More often, they also exhibit reduced growth rates and disease resistance compared to freshwater fish. Think of it this way: a freshwater fish has had tens of millions of year to optimise its biology to freshwater habitats; a brackish water fish is still at an in-between phase, and isn't optimised for life in freshwater habitats. So they waste some energy compared to freshwater fish, or aren't as able to conserve minerals, or whatever. Overall, they're less good at living in freshwater.
Brackish water fish are "jacks of all trades, masters of none". They thrive best in those variable habitats where being adaptable helps. In either purely marine or purely freshwater habitats, marine or freshwater fish outcompete. That's why brackish water fish are restricted to those narrow regions along the coastline.
So, while doubtless hardy enough in freshwater for weeks if not months, long term, it does need a brackish water aquarium. These are great fish, and I kept them back in the 1980s, when they were widely sold in the UK. As adults, they are big but peaceful, easy to tame, and entirely omnivorous (so no small tankmates, and regular offerings of green foods).
Cheers, Neale
these two gobies were caught off a canal in broward county in purely freshwater. i'me being told now that I had caught these previously at the dipnetting competition we had, and the collection location at that point was about 5 blocks again in the same canal system. again, purely freshwater...so what gives? in NANFA i was told they same thing, Dormitator maculatus, and was shown a picture of an adult fish and it matches the ones i caught.
