Many consider the "MSU RO" or "Akerne's Rain Mix" to be good fertilizers. If we assume the Mg content was specified as being appropriate for the 125 ppm N dosage they recommend, then that is the equivalent of 0.71g/gal or 0.19g/L of Epsom Salts applied at every watering.
Usually I use half that dosage.
But I've watered in the pasr with a far higher Mg concentration without any issues.