Is tap water OK to use? Does it matter if we have hard water or a water softening system?

Yes, tap water is fine. Hard water does not affect the detox process. (Here at the home of the  Ionic Body Balancer™, we only use warm/hot tap water straight from our bathtub. We have very hard water and have found that hard water does not affect the detoxification process.)  However, if you have a water softening system in your house, you will need LESS salt. Water softening systems add sodium (salt) to your water to negate the effects of other minerals in the water.
If you have a water softening system, or if your local water company adds salt to the water supply, you may find it helpful to keep a bottle of distilled water on-hand to reduce the salinity of your foot bath water. You will need to heat the water prior to adding it to your foot bath. You can contact the manufacturer of your water softening system if you have questions about the amount of salt that has been added to your water. Due to the wide variability in these systems, we are unable to answer any specific questions about any water softening system you may have installed in your home or workplace.
If you live in the U.S., you can find information about your local water supply on the EPA’s website.