You should only ever use distilled water, and nothing typical tapwater, because of all the impurities still remaining in tapwater, including chlorine.
Distilled water is as pure H2O as you can get without going to a lab.
Response From by: Hydrogen-Generators-USA.com
The only type of water you can use with a hydrogen generator is distilled, steam distilled or deionized. Regular tap water has minerals in it that interrupt the electrolysis process.