Be sure to maintain very clean propagation vessels and change your water frequently to keep it fresh and clean and to replenish oxygen. Once your ZZ plant propagations in water have roots (whether you started off with leaves or a stem itself), go ahead and plant in potting mix. You can even wait for a rhizome to form, but don't wait too long. If you leave water propagated ZZ plants in water for too long, they may eventually turn yellow and rot.