I use well water in NW North Carolina. My unfiltered well water tests at about 0.020 on a portable TDS meter and contains quite a few granite particles (I believe they call this well fines). Keep in mind that the fines will eventually affect/clog a RO/DI prefilter, depending on how many fines are in the well water. If your well/house is equipped with a filter, then this shouldn't be a big deal, but I hear from a few sources that you should still use an RO/DI device on well water, and not put well water directly into your aquarium (due to possibility of organic contaminants, pesticides, microbes, bacteria, leach field contamination, etc.). Side note: I've heard a few people talking about using the new Zero Water pitcher (which claims a 0.000 TDS reading) for use in small amounts of water preparation, like for a nano. I have been testing the Zero Water pitcher theory on a 40 gallon reef system with no adverse side affects so far (testing is 2 months in progress).