Buffering RO/DI water

[DocBob]

I recently moved to a new place and my water quality here is terrible! I am going to install a RO/DI system, but had a couple of questions.
I have heard that when using RO/DI water for freshwater tanks it is typically used in combination with some amount of tap water to help buffer the pH. I currently have a continuous water change system set up on my 180 gallon stingray tank, and was wondering what options I have.
Can I run the RO/DI water directly into the sump as I currently run the tap water, or should I catch it and add tap water to it before adding it to the tank?
If I were to use straight RO/DI water with no tap water added, how do I buffer the pH?...or do I need to actually worry about it at all?
Thanks for any input you might have.

[Fruitbat]

The problem with RO/DI water is that it contains few, if any, of the minerals that act as pH buffers. This allows the pH of the tank to change RAPIDLY and sometimes catastrophically for the fish. As long as you can avoid pH shifts then there is no problem with using RO/DI water exclusively, though it will probably stay on the acidic side due to carbonic acid build-up (C02 reacting with water).

The mix of tap water and RO/DI water is probably a safer way to go, especially considering how sensitive stingrays are to water quality.