Epsom salt (Magnesium sulfate) is an inorganic salt (chemical compound) containing magnesium, sulfur, and oxygen. It is commonly called Epsom salt, taking its name from a bitter saline spring in Epsom in Surrey, England, where the salt was produced from the springs that arise where the porous chalk of the North Downs meets non-porous London clay.
Epsom salt raises the general hardness of the water. It does not appear to have a direct impact on pH (aside from providing pH buffering). Epsom salt is also a mild laxative and muscle relaxant and can help with swelling, constipation, pop-eye, and dropsy. A dosage of 1 tablespoon per 5 gallons raises the hardness to around 20 dH and is well within the tolerance of many fish and almost all invertebrates found in the aquarium trade. Epsom salt on its own will not cure pop-eye or dropsy, and the underlying causes of the problem (like high nitrates or physical damage) need to be addressed as well. In the case of dropsy, antibiotics may need to be used in addition to Epsom salt.