RODI is only for SW tanks. His is a FW tank. If you used RODI on FW you would have to add elements back in that you are removing. In SW we add salt mix to the water to create the levels that we want, and only water evaporates, and we add other things like Alk Cal Mag etc to keep parameters where they should be as corals and organisms take these out of the water. If you topped your tank off with pure RODI in FW you would mess up your pH