Salts (chemistry) should not be confused with salt as in table salt, sodium chloride (NaCl).
Fortunately
Epson salt, also known as Magnesium Sulfate (MgSO4·7H2O) is not toxic at all as it is a compound that is contained (in large quantities) in literary every
nutrient solution, except some "bio", where Mg is provided through Magnesium Carbonate extracts. Many other compounds of a nutrient solution are actual salts.
If the Mg content and the Sulfur content (epsom salts contains much Sulfur) of a nutrient solution is sufficient, there is no need to add any epson salt.