I use tap water. Every Nutrient maker will tell you that you have to change out your rez every week. It just makes sense for them to tell you to dump their product down the drain and buy more. Even Lucas, (if you don't know who he is, find out bro), has stated that scheduled rez changes are unnecessary. I like to experiment, so I'm doing a second consecutive grow just to see the negative impact, and guess what? Thats right. no negative impact. I got about a week to go and everything is coming up roses, so to speak. I am not "down" on anybody who wants to pour money down the drain and spend hours going through the hassle of doing constant rez changes, it's just not for me. I've only done about 12 to 15 grows without changing my rez, maybe I've just been lucky..., and then maybe I have never been given any scholarly reason, backed up by definitive proof that you have to. I have a background in wood science and botany that precludes me from taking anecdotal advise without experimentation. It is this same reasoning that keeps me from adhering to the "rinse-repeat" command on my shampoo bottle. Once seems to do the job pretty well and no one has ever told me "WOW, it looks like you only shampooed your hair once, you forgot to repeat.". I do a lot of things in an unconventional, or counter-intuitive style, but I am only interested in results, not approval from my peers. All I can suggest is, gather information, and trust your gut. There is a lot of what I call, "conformation by repetition". If one person says something and it's seconded by another, then all of the sudden it is believed as fact. Lots of good advise on this site, lots of bad too, trust your instincts.