When I first started passaging my starter, I used tap water. I’ve since done a lot of reading to try to improve my skills, and found that the type of water used affects starter performance since tap water can contain inhibitory levels of chlorine. In my first experiment to test the impact of the type of water used, I split my starter into purified water and tap water (control).
first test for purified water

The starter with purified water had a much shorter lag than the starter with tap water. Since then I’ve switched to using purified water when maintaining my sourdough starter. Some sources suggest boiling tap water or letting the chlorine off-gas overnight before using. I haven’t tested these methods, so cannot confirm their effectiveness.
Chlorine is used as a disinfection agent in water treatment plants. The 2015 Water Quality Report from my town reports that the maximum concentration allowed is 4 ppm, and the highest level measured was 2.67 ppm. The minimum inhibitory concentration for chlorine of typical industrial yeasts range from 10 – 125 ppm. While the levels observed in my tap water are well below these limits, I do see a difference in the two cultures. The starter with purified water gets to a higher volume quicker, and was more active over the 12 hours.

See for yourself here:

Do you use tap water, or purified water with your starter?