Ionic foot bath are most effective when a person is more acidic, thus making the water "blacker"....as we do more foot baths the water gets cleaner, but that doesn't mean that we are clean....we may be cleaner as a result of the foot baths but not clean.....the water looks cleaner, because our body pH becomes over time....closer to the pH of the salt water.
I add a pinch of baking soda to my foot bath water to create more of a 'pH differential" between me and the foot bath water, thus allowing more negative ions to enter my body through the foot bath.