RO/DI is Reverse Osmosis and DeIonized. It's pretty much 100% pure water. I have an RO/DI unit for my saltwater reef tanks also. I was actually worried a bit about using it for brewing as I would have to add a ton of salts back into the water, so I thought it counter productive. However, I do carbon filter my RO/DI unit to .5 micron before the RO membrane, so at that point it has scrubbed off a lot of salts and all of the chlorine. So I've tapped the RO unit before the membrane so that I triple filter my brewing water to .5 micron, then add 5.2 and forget about it.
-J