When i first started my FOWLR i used tap water and a dechlorinator. Of course, I moved into the coral side of things, (Who can resist!?!!!) and continued with tap water and generally speaking, weak lights and irregular water changes, i was (and still am) a beginner.
CR Reef member "Option" has a beautiful reef tank and he told me he uses tap water and trust me, he has amazing growth on softies and lps! But I don't know the specifics of his tank, equip, and husbandry practices other than that he uses M.H. and a sump. Personally, I would just get a 4 or 5 stage rodi system from BRS with a tds meter attached.
Since 3 months ago, I have gotten a 5 stage brs rodi system, it takes a few hours to accumulate the clean water I need and it uses up pricey media, so I can't just waste it, therefore I use a black colored 32 gallon rubbermaid roughneck plastic trash container from either home depot or Lowe's (i don't remember), since its made from a safe plastic, to store my clean rodi water. The brand doesn't matter, what matters is the number on the bottom or inside of your container, I suggest you do a little research on food safe plastics. I keep a heater, thermometer, and an old powerhead in it to make smooth top offs on my tank.
AFTER a few months of using rodi, and highly upgraded lights, I can see some definite improvements. (went from 216 watts to 716 watts). I am also 1,000% positive that using rodi has contributed to the slow down of the algae growth in my tank, I also don't have to use dechlorinator anymore.
2 cents