I don't mean to play the Devil's advocate here, but there are plenty of threads where hobbyists are using RO/DI and still have problems with cynobacteria. I've been using tap water for almost 14 years now without any problems. You might want to check out that link that I posted earlier. There's no way that everyone was using tap water. At the very least, just make sure that your tap water can't be used first. It would be a damn shame to muck up RO/DI. GL.
http://reefcentral.com/forums/showth...highlight=cyno
http://www.advancedaquarist.com/2004/1/chemistry