ancient thread, but had to chime in as it's referenced in the "101 already answered questions" and a few things are missing. "tap water is fine" as a blanket statement is false. my city tap water reads a consistent 1.35 on an EC meter, which translates to 675-945 ppm depending on your meter's conversion scale. it's high in sulfur and sodium and smells like farts, not suitable for human or plant consumption. plants watered with my tap water perform like shit. as for the chlorine thing, it is very volatile and does evaporate quickly, and it is typically found in concentrations too high for healthy plant growth in many city tap waters; however, it is an element necessary for plant life, albeit in minute concentrations (like sub-30ppm). as i dread the sodium and sulfur toxicity i get from local tap water, i also add both elements back when i mix up a hydroponic reservoir as they're both necessary in minute concentrations as well. all tap water is not created equal, some is awesome and contains enough calcium and magnesium to avoid deficiency without any supplementation, and contains proper levels of several necessary trace elements; some comes out of the tap visibly yellow like mine and i won't let that shit anywhere near my garden. i buy RO systems on ebay for around $110 shipped with an extra set of filters, 6 stages of filtration and my 1.35EC water comes out 0.00EC, typically measure around 0.05 after 6 months on one set of filters. worth every penny.