We had a well installed several years ago and at the time I did not have an RO unit, and was still relatively new to the hobby. I started noticing ALL of my inverts start to very very slowly deteriorate, and NOTHING would check out. I was under the wrong assumption that the well water was probably the best I could get compared to being on the public water system. I finally tested the water out of the tap, and bingo, copper. It was a very very minute amount, but over time with top offs, and water changes it gradually accumulated in the water. I wouldn't risk it personally.