Saying a brackish fish can adapt to either fresh or salt is a bit of a stretch. Striped bass, for example, can adapt to either, as long as they have fresh water to spawn in. However, a close cousin the white perch, needs to have either fresh or slightly brackish, but can't tolerate full saltwater for long. The atlantic stingray lives in full salt, but can live in brackish, and even tolerate some freshwater, but they cannot live in freshwater for long. Many marine species have young that live in brackish, and then return to the sea when older, like some salmon.
Anyway's the point is that some fish need different conditions than others. Mollies are a true brackish fish, they need some salinity, can tolerate full fresh or full salt. But they really do best in brackish. I've never kept the silver tipped shark, but I think I read somewhere that they live in fresh to slightly brackish water. Monos, scats, archerfish and some puffers are examples of fish that really need a little salt to thrive.
Anyway's the point is that some fish need different conditions than others. Mollies are a true brackish fish, they need some salinity, can tolerate full fresh or full salt. But they really do best in brackish. I've never kept the silver tipped shark, but I think I read somewhere that they live in fresh to slightly brackish water. Monos, scats, archerfish and some puffers are examples of fish that really need a little salt to thrive.
