Thanks for the responses!
Waterbear: I'll have to look into the correct concentration, but if my memory serves me the concentration they tell you to put into an Epsom Salt bath is roughly between 400-600grams per 600L, which would be about 10Kg on a 400Gallons hot tub if you went on the lower end with 400grams. That is quite a bit. The baths were for 12 minutes at 50-55 degrees celcius, which I believe is warmer than most tubs.
chem geek: wow very techical, so laymens terms......adding magnesium shouldn't be a big issue? I guess it would depend on how much I add right?
spidey9: Yes you are right, that amount of magnesium probably wouldn't do much at all...when I think they are talking about putting 10Kg into a 400G hot tub.