Some freshwater species (and the blacktip shark, a marine species) are hardy enough or survive better in brackish water, such as Polypterus bichir, certain loaches, Danio rerio, all kinds of mollies but especially the Yucatán molly, and some gobies. All can tolerate the same amount of salt in aquaria, but should be acclimated slowly.
In the context of aquarium-kept fish, certain species such as Polypterus bichir, loaches, Danio rerio, mollies (especially Yucatán molly), and gobies can tolerate brackish water conditions. However, they should be introduced to their environment gradually.