I decided on the tap water because the guy at the LFS
uses tap water for his own reef. I did test before and after mixing, and no nitrates at all. I have a chemi-pure type bag in the sump for phosphates and silicates.
I'm believing that the old system was nitrate high. The previous owner said they rarely did water changes because "the system kept them low." But I don't think that's the case. I think that when we kept some of their water, probably 100 gallons, it concentrated the nitrates and they've always been high. I'm going to do daily water changes and see where that gets me.
But I sent you a PM, I'd be interested to see your thoughts.