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.
Some fish species, such as Polypterus bichir, loaches, Danio rerio, and mollies (particularly Yucatán molly), are tolerant of brackish water conditions. These species can coexist with similar levels of salt in aquariums but need to be introduced gradually.