I don't know what the tap water is where you live. Typically, it is treated water and conatins a variety of things, like fluoride, chlorine, and metallic salts. That will kill many CP's, over time. Boiling the water, as in what some do for traopical fish, would evaporate the chlorine, but it would concentrate the metallic salts, making it worse for the plants. Not sure of how much water would be required, but as long as the plants are completely submerged, you won't need to use any more the water. When I was doing this, I had the equivalent of 2" pots, so I didn't need any more than a pint. Can you use distilled or rain water or stream water?
I would submerge them for a day and then see what the results are, perhaps with a magnifying glass. If you see floating whiteflies, then it was probably sufficient. I know with aphids, a day or two was all that was needed. Keep us posted
With regard to rain water that runs through gutters or off roofs, I tend to think that anything that is quickly running along the surface won't pick up much in the way of anything harmful. Now if it sits in a spot, I would be dubious. I have my trash barrels staged right where the rains runs off the roof and it hasn't seemed to cause any problems.