Distilled water is pretty good, but it is not as pure as RODI. In our lab, where we grow cells in culture, only RODI is used. That said, distilled water is infinitely better than tap water for tank use, and probably only slightly less good than RODI for this use. As long as you keep to a regular water change regimen you will be successful, even with tap water. Use the distilled - it will be fine.