I don't even think survive. #1 osmosis is not the only thing fish use their surrounding water for. There is ion-exchange and charge balancing, too. A fish is accustomed to a certain amount of minerals in the waster and their ion exchange and charge balance rates have evolved over millions of years in response to their natural waters. Now, they have a certain amount of variability in case of emergencies, but that is more survivability than long-term. Using a mix of the two to make brackish water only means that both freshwater and saltwater fish are well out of their normal parameters. Too many minerals and not enough minerals, respectively. Also, for freshwater fish, the extra minerals will cause internal organ damage (kidneys, livers) because those organs have evolved in freshwater and comparatively mineral-poor waters.
All-in-all, nature just doesn't work that way. You can't breathe in an environment with 50% less oxygen or 50% more oxygen. We've evolved precisely to the amount of oxygen that is in the Earth's atmosphere, and there isn't much room for variability. it is the same for fish, they have a very small window of correct mineral content and that's it.
Before I would believe any kind of the claims that would be made, I would have to see peer-reviewed scientific papers backing the claims up and at the very least some proposed mechanism how the normal biological processes can still continue in the wrong environment.