Actually, what happens with RO/DI is that it has 0 KH. KH is what hods up pH. So when one makes RO/DI it should be dead neutral. But it is not, it actually turns acid pretty fast. This is due to the carbon dioxide which the water naturally absorbs from the air. This in turn creates some acid in the water. Since there is no KH, the pH of the water drops.
I use a digital testing for my one RO/DI added tank. I mix 20 gal. of changing water which is 11 RO/DI and 0 tap (7.0 pH and 83 ppm TDS). The testers really hate pure water in terms of the TDS (conductivity). I never put the probes into the changing water until I have got it all mixed save for any need to lower the pH with muriatic acid. This may be done based on where the tank parameters are when it its time to do the water change. Between water changes my water moves up from the 5.8-6 pH I start it at towards 6.5.
The most important consideration has to do with the bacteria and the cycle. Despite what one often reads on fish sites, it is not true that nitrification stops if the pH falls under 7.0. What happens in such cases is that the bacteria have to change their source of energy from ammonia (NH3) to ammonium (NH4). As the pH drops, the ammonia (NH3) levels head towards 0 and all the ammonia is in the form of NH4. The bacteria mostly have receptors for ammonia, but they also have some for ammonium. So they are able to switch over. However, they do not process ammonium with the same efficiency. This process takes time and, in the normal cycle, it will appear that the ammonia have stopped working entirely.
However, a bit of patience and one sees that the cycle may be interrupted, but that it then resumes. Since most fish keepers are not working with water much under a neutral pH, what they see is a stalled cycled. The one side benefit to all of this is that ammonium is a whole lot less toxic than ammonia.