For the first 3 years I kept a reef tank I used tap water, nothing added other than dechlorinator, flouride is added to the water here, and the only ill effect I noticed was a little more algae growth than is present now with the use of Ro water and I contribute that to the various nutrients. Im no chemist, but I dont know of anyone around here having problems with using tap water in both marine and freshwater tanks. Of course RO water is still optimal.