Yes, unless there is something in the tap water that you do not want in the tank, the cheapest and easiest solution is to use some tap water with the RO. However, as Quantum pointed out, cardinals do not need any of this, so be careful.
I never buffer pH, my tanks are at 5 or perhaps less, no test kit I can find goes below 5. I do however have to add some GH due to the plants, as Quantum mentioned. There is insufficient calcium and magnesium in prepared fertilizers, since they assume tap water will supply most of these. My tap water is near zero GH and KH.
On a related item, I just posted this photo in another thread so thought I'd stick it in here too, as an example of a very authentic cardinal tetra biotope; more leaves on the substrate is about all I would add. Dim light, essential for cardinals as they have a "light phobia." The second photo is a specific biotope of the Rio Huallaga in Peru.
Last edited by Byron; 01-24-2012 at 03:40 PM..