You should never add Epsom salts to your spa or swim spa.
Epson salts and Magnesium salts will alter the pH of your spa water and salt levels above 1500 PPM and can be corrosive in your spa.
Unchecked, this can cause a chemical reaction that will attack dissimilar metals. In a Spa Pool, this is quite often the spa heater element or parts of the spa pump.
High pH will also prevent your sanitiser from working efficiently and may cause cloudy water.
So we suggest using your Epsom salts in your bath tub or foot bath where you drain and change the water with every use not in your spa or swim spa.