African Dwarf Frogs live in both fresh water and water with slightly higher salt levels. It is advisable to not keep frogs in water that has a high salt level – the more salt, the more frogs can’t stand it (it’s like too much salt in your blood, you’d feel sick). The ideal water to use is around 5-10 ppm. I don’t know the exact reason why a high salt level is bad for the frogs, but it is. If there are too many chemicals in the water, the frogs will get a fungal infection or parasite infection. The good news is that African Dwarf Frogs are used to living in few places, so they are very tolerant to different water types. I believe that the frog will die (unless it’s a baby frog) if you put it in freshwater because they can’t breathe. So make sure that if you keep the frog in freshwater, it has some way of breathing – either a vent (a hole on the side of the tank), a floating mount, or something that allows the frog to at least stick its nose out of the water.