The problem is, if you start with tap water, its gonna take months to actually remove all the tap water and replace with R/O water, because you're only diluting the tap water down a fraction, every time you replace with R/O.
plus, you can run into weird water chemsitry issues, some of which won't bother fish, but will affect inverts and especially corals. I started my 40g SW
with tap water. Then I had weird low pH levels, semi-high Alkalinity, and calcium was half of what it should be.
So prior to adding fish, I did a 10g w/c with R/O, and that helped my pH a little, but for several months I still had low calcium and normal alkalinity, even when I'd add calcium.
I tested Magnesium and it was at a proper level, so it wasn't that which was affecting my chemistry.