I am in the market for water purification but am trying to become an informed consumer first. It seems that the best system from what I've read is an RO/DI. However I don't really like the idea of wasting our most valuable resource and correct me if I'm wrong but RO systems can waste 90-95% of the water entering the unit. Since DI systems apparently are 100% efficient, wasting no water, why not use one alone? Is it that the final product is not pure enough? Which raises the question, for a reef tank how pure does the pre-mix water have to be? How much does this requirement change for a fish only/live rock set up which is what I have at the moment (planning to start with coral as soon as I can afford the lighting!).