You should all read my article on hardness and pH in the freshwater aquarium. Briefly, water is "hard" due to the level of dissolved mineral salts in the water. "Salt" as we use it commonly refers to sodium that you use in cooking, and related saltwater as in the ocean is this type of salt. But calcium and magnesium salts along with some other mineral salts make water hard according to the level of mineral salts in the water.
The rift lake salts are mineral salts. They will work for rift lake cidhlids if you have soft water, but there are less expensive ways to harden water. Dolomite is the easiest as it is calcium and magnesium, the two prime minerals for hard water.
"Epsom salts" is strictly speaking Magnesium sulfate, which is simply a mix of magnesium and sulphur salts. I add this to my tanks to give me a tad of mineral, plus it is excellent plant food. But you have to be careful, too much causes other problems like iron deficiency. This is still strictly freshwater, no sodium is being added.