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