Tap water is unacceptable for use when mixing salt mix for use an any saltwater fish tank.. City tap water quality varies from site to site and from times of the year. There is a huge amount of added chemicals (chlorine, flourine, chloramine, etc) AND dissolved phosphates, nitrates, silica, sediment, debris, solids, junk, & crap that "good" city water contains, , ALL of which does not ever belong in your tank.. Planning on using tap water on a saltwater tank is a sure recipe for failure..
it's amazing to change out the 5 micron particulate filters in RODI units, that's when you get to see the slime/mud/muck/dirt & crap that the unit filtered out. Most folks gotta change these every 3 months or so.
There's alot of junk in ANY "city" water. Again, none of it belongs in a salt tank.