I'm with Sarah on this one - although I use a plastic dish pan
I use about
a gallon of water with those amounts of the Epsom salt and peroxide.
If the foot spa has a heating element or anything like that where the water would come in contact with it, the Epsom Salts may corrode it. A bucket or dish pan would be better at that point. Save the foot spa for a night you don't want to detox, just relax.