DCF I really don't think you should use tap water unless you test for all the metals and there aren't any. I just know from like my moms well her iron is "low" says the people who dug it. But it has a metallic taste to it when she doesn't run her ro.di. Plus I tested my "ro" water I had been getting from the grocery store last night cause my phosphates are on the rise, and it didn't show at zero. I guess you need the "di" part to get all the nasty stuff out. Remember I am new at this so this is just from recent experience. Good luck!
-Heidi
As Jimsflies (a specialist in drinking water) has stated, it depends on the source. Several people have successfully used "tap" water from a well, while others are not able to do so. Every well source water has minerals and, potentially, other undesirable contaminants. Michigan water is high in iron, sulfates and chlorides. Some areas in the state are high in nitrates and phosphates.
Unless you have your water tested, I would not use it in my tank and put my livestock at risk.
BeakerBob - Past MMMC Club President, current Board Member