Think of the big picture first Tony then all the details will fall in place.
Fish and other vertebrates like us have a common characteristic. The salt content of their blood is identical (0.9% salt solution). This salt or electrolyte is composed of many different substances or compounds. About 77% of the salt is sodium and chloride in our blood. Only 1% of this total salt group or content is Magnesium (Epsom salts). Therefor it is of greater need to re-supply the blood when leakage occurs with NaCl (solar salt) than it is the magnesium (Epsom salt).
The next time you get blood work done, note the amount of salts or electrolytes that are measured. Remembering koi have the same salt solution percentages in their blood as we do . For example:
Sodium 135-145 (mmol/L)
Potassium 3.5-5.0
Chloride 98-108
Bicarbonate 22-30
Magnesium 4.5-6.0 This is an educated guess