With the obvious expections of Cholrine and Chloramine fish can handle Tapwater so long as Nitrates can be kept at 20ppm or lower - Corals + Inverts not so much as they don't do well with nitrates over 5 ppm. Problem with SW tanks and Tapwater is more or less undissolved solids that decomp + added phosphates from the tap which can cause + seriously fuel obnoxious algae + bacterica blooms. However SW fish don't handle PH swings very well so large volume w/c's isn't recommended for nitrate control.
With that said theres a million ways to skin a cat - Algae scrubbers, Cheatomorpha Fuges, Gac Reactors, GFO reacrors, Carbon P Reactors, Protien Skimming, Nitrate absorbant media, etc..... can all help purify intank water. Just gotta pick and choose your finacial battle and with from the Tap + corals it's all uphill - not really gonna be a big issue for fish.
RO/DI water is 99.8% pure h2o - A 100gal a day unit cost like $250 with no real significant increase in the water/sewer bill or electric bill incase you decide to go reef in the future.
