Different ionic foot spas have the ability to give of different ratios of positive and negative electrons. Some give off ONLY negative ions and separate the positive. Some give positive and negative ions but mostly positive. The models that most people call me with the best feedback with are usually the models that only give negative ions. Most people are acidic with an abundance of positive ions in their body.
I personally wait about 15 minutes after the ionic foot bath is turned on and then soak my feet. The longer the water is charged up the more negative ions the body can soak up and the more pull the soak water will have on the toxins that are neutralized in the body.
To get the highest polarity of negative ions my best advice is to get an ionic foot bath with as little ripple as possible or no ripple at best. See my page on Ripple effect on ionic foot baths.