That's a fantastic question and I have a fantastically unsceintific answer for you that is basically based on a mixture of trial and error and pure stinkin' luck.
I use both.
As you two know, my nano tank is at my office. The office has RO water available through RO drinking water filters. I mix RO and the tap water in about 30% tap, 70% RO ratios.
Christin, this will apply more to you because we're both in San Diego and guitardude, sorry, but I don't know the composition of the water in Chicago. San Diego has really hard water. As a result, I started mixing the water with RO more to soften it up a bit than anything else. This also, as you know, affects your PH. It took me about 3 weeks to figure out what ratio got me about the right PH over time and I tested it a few days ago preparing to match the PH in my office tank with my home tank to bring some CRS home and it's still right around 6.6.
Like I said, trial and error and luck.
I have used straight RO water in my home tank and decided that it was more trouble than it was worth. When I did I used Seachem Equillibrium to reconstitute the water. I've since starting mixing RO and tap water at about the same ratio that I do at the office.
And it's all "by eye". I don't measure it out. I don't test the KH, GH and PH everytime. I've never had a problem and though it's competely unscientific I plan to continue this way. That's where the luck comes from.
