If you are using hydroponic
nutrients (as in the botanicare line), there is no benefit to using Epsom Salt in your
nutrient solution. Actually, if doing so you will through the balance of the nutrient solution off. Hydroponic nutrients contain both Macro-elements (nutrients consumed by the plant in large quantities), and Micro-elements (nutrients consumed by plants in small quantities, also called "trace elements"). Nutrients that are made for soil gardening rarely have any Micro-elements. Some people use fertilizers designed for soil in hydroponic systems, and they sometimes use/add Epsom salt to the nutrients as a source of the lacking Micro-elements in the soil fertilizer. That isn't necessary or even beneficial if your nutrients were designed for use in hydroponics, hydroponic nutrients contain the Micro-elements, and in balanced amounts. Adding this to your hydroponic nutrients will just through off the balance of the Micro-elements in it.
Nutrients - Over and Under Use
Nutrients: Beyond Macros and Micros