According to this vet, "Be sure to not use distilled water, which is devoid of minerals and salts, and is dangerous to the osmoregulation of frogs and toads. Other types of acceptable bottled water include filtered water or deionized water that has had minerals replaced. Read the labels on bottled waters carefully to ensure that you get one with minerals and salts added (or not removed in the first place)." American Toad Care and Husbandry (Bufo americanus)