I know you will not like what I'm going to say, but I'm a trained IBM Engineer so I know how this works.
Here it is: Ionic Foot Baths are a scam to get your money. The story is "Look at how dark the water has become after taking an ionic bath" Yes, that is a fact, but they LIE when they say that it is caused by toxins removed from your feet.
It is really caused by electrolysis between the metals used. The water is salty so it conducts a current between the metals, thusly turning the water yucky dark.