I`m one of those that think you should not use tap water. Esp. for a reef tank. I work for a large water municipality. I know what goes in there for clean drinking water. Our water is rated the third best in the country so it`s great water but for human consumption. The compounds added to make it clean are good for us humans but bad for our reefs. Things like nitrates, phosphates, flouride, chlorine and many heavy metals used to bring the dirt particles to the bottom. Not a very good recipe for our reef tanks. Maybe the initial fill might be OK but none after that. I`ve had my reef for almost 13 yrs now without a crash yet (knocking on wood) and I`d like to attribute some of the cause to filtering out all that stuff with my RO/DI. Just my opinion and experience.