Epsom salt is the common name for the compound magnesium sulphate — a chemical containing magnesium, sulfate, and oxygen. Despite its name, Epsom salt doesn’t contain sodium. Magnesium is a mineral that is important for many systems of the human body — especially the nerves and muscles. It’s also beneficial to aquarium fish suffering from maladies such as dropsy, constipation, and swim bladder disorder. Epsom salt may be added to fresh water fish tanks to alter the chemistry of the water.

Epsom salt may be used to increase water hardness in freshwater aquariums. Harder water contains more dissolved magnesium and calcium than soft water. Some species of fish require hard water, while others require soft. The degree of hardness impacts the tank’s pH. Hard water is more alkaline, soft water is more acidic. Water hardness is expressed in the amount of calcium carbonate (CaCO3) it contains. According to Aquatic Community, “adding 1 milliliter of [epsom salt] per 10 liters of water will increase the permanent hardness by roughly 70 mg/L CaCO3.” Determine the optimum pH and water hardness levels for the type of fish in your aquarium before altering your tank’s chemistry. Abrupt shifts in pH levels can stress, even kill fish. Avoid a change in pH greater than 0.3 in a 24-hour period.

Constipation is a common condition suffered by many types of fish, including bettas. Symptoms include belly bloat and loss of appetite. Constipation is usually caused by overfeeding or feeding a low-fiber diet. Stop feeding a fish with this condition for 24 to 48 hours. If you don’t see improvement, offer the fish tiny pieces of vege based food. If the fish shows no improvement, consider giving him an Epsom salt bath, which acts as a muscle relaxant. To give your fish an Epsom salt bath, pour half of the tank’s water into a clean container. Add 1 tablespoon of Epsom salt for every 1 gallon of water. Have the fish swim in the solution for 15 to 30 minutes. Remove the fish promptly and return him to his aquarium if he appears stressed or relieves himself.

Dropsy is a potentially fatal condition in which the kidneys of a fish fail, leading to fluid retention. A fish experiencing dropsy has a bloated abdomen and raised scales. Epsom salt may be used as part of dropsy treatment by isolating the sick fish in a separate “hospital” tank, then adding Epsom salt at a rate of 1/8 teaspoon for every 19L of water. The Epsom salt will relieve swelling.  Never use aquarium salt, or any other type of sodium, to treat a fish with dropsy: It may worsen the condition.

Swim bladder disorder can strike all freshwater fish, but goldfish are especially prone to develop it. Fish with this condition have difficulty swimming upright because the swim bladder, which is normally filled with air, is compromised. Swim bladder disorder may be caused by gulping too much air, impacted eggs, infection, or constipation. Treat swim bladder disorder by skipping feedings for two to three days, then feeding a vege based food. If that doesn’t help, increase the tank’s water temperature to between 26 and 28 degrees and add Epsom salt at a rate of 1/8 teaspoon per 19L of water.