Well, I didn't actually test it. I looked it up online, and the water table said that the measure of copper doesn't exceed the 0.05's range. I'm in Hollywood, Florida, and I know a lot of people around here use their tap water and are fine with it. All I'm using is dechlorinated water, and everything appears to be doing well so far. I know that my parents would -not- go for an RO unit; I already tried to talk them into that, even with my raising money to buy it myself. Not happening. But the good thing is, this is my first salt tank, and it's not that big, so if I really mess it up, it's not -that- big a deal... we're planning to go bigger once we "conquer" this size, lol. Something like 300 gallons. So, if and when we get there, then the RO unit will be re-discussed, because I'm not going to risk something that huge.