Can you give an American toad tap water to soak in?

Yes. You can. But I would advise to let it sit in an open pot and air for at least 24 hours to let the chlorine content it contains be allowed to evaporate. The same goes for fish tanks. High levels of chlorine can be harmful and even deadly to reptiles and fish who are not accustomed to chlorine in their environment. Use rain water if you can.