[Bogie]

4 parts Arm & Hammer to 1 part Borax.
And, Wildreef, I'm sure you've read, but doubt you understood what I wrote.

The concentration of Magnesium needed in our tanks 1300-1500 ppm, is much less than the concentration of epsom salt used for a foot bath. The Magnesium concentration (ppm) contributes less than 4% to the overall specific gravity of the water in our tanks.

Here's a quick picture:
A 100 gallon tank requires 50 cups of salt mix to bring the salinity to 33 ppt, with an SG of about 1.024.
This 100 gallon tank may only require 2 cups of epsom salt to bring the magnesium up to 1500 ppm (depending on which salt mix, ect...), and might raise the total SG to 1.025.