I am no expert but I am pretty sure I read about populations of bull sharks that live there whole lives in fresh water. It doesn't really matter though cause it would not be practical to keep one in freshwater. I think that for most people, it would be very difficult to keep in a regular marine aquarium due to the size, aggression, and not to mention the price. Your going to need a lot of Benjamins to buy and transport one of these guys. But if you can pull it off by all means go for it. Thats just my take on it.