Aquarium salt is a type of purified sodium chloride (NaCl) used in freshwater aquariums to supplement essential electrolytes, improve gill function, and reduce stress for fish. It can be used for general health maintenance, such as during water changes, or for treating specific ailments like nitrite poisoning or external parasites. However, it should be used with caution around live plants and sensitive, scaleless, or soft-water fish species, and is different from marine salt, which contains trace minerals for saltwater tanks.