While you can use tap water, it is better to use distilled water because that eliminates most of the impurities in normal water.
We also provide you with several catalyst options for the electrolyzer to add to the water in order to obtain the greatest HHO (hydrogen or Brown's gas) output. Baking soda is one very safe catalyst, but there are also others that work very well and in some cases, even better.
Ryan
Run Auto With Water Mechanical Support
http://www.runautowithwater.com