Looks like we're toeing the line between spirited debate and flaming here. I'd love to leave this thread open as this is quite a debatable topic and some great info/ideas have been said in it. If it does turn south though, well we all know what happens then, try to keep things civil guys.
Having said that, this argument is very complex and unique to each individual person. All of our tapwater is unique as it all comes from different sources and contains different elements in it, thus influencing the choice of using RO or not using RO. We also have to remember that certain animals have documented incompatibilities that MAY be found in someone's tap water, but not someone else's. Obviously if you intend to keep an animal with documented problems with something in your tap, RO may be the choice for you.
I can say for sure that little to no longterm science has ever been performed on what "trace" minerals sometimes found in tapwater can be detrimental to fish. Sure we know chlorine/chloramine, bromine, and flourine (common disenfectants), as well as ammonia are certainly toxic to fish and sometimes found in tapwater but these can easily be removed with carbon filtration, dechlorinator, or just leaving the water to sit and allowing them to break down and evaporate into the atmosphere. Then you get into the gray area of metals like copper or iron. Sure, copper concentrations over 0.3ppm is toxic, we know this, but finding copper in tapwater close to that concentration is very very rare. Usually it's in the parts per billion concentration, at which point even irregular water changes can take care of it. Iron is present in seawater at a relatively high concentration, so again, toxic levels of iron will be difficult to achieve via tapwater.
As far as I know, little if any research has been done on sulphates (common in well water), same goes for silicates, but speculation abounds that both may help fuel cyanobacteria blooms. And then there's nitrates/phosphates, again, different for every system.
Having said all that, lets examine my own tapwater. It has high concentrations of chloramine, flourine, silicates, calcium, and carbonate/bicarbonate (double digits in parts per million). Small concentrations of sulphates and magnesium (single digits ppm). Trace concentrations of copper, iron, and a myriad of other salts and metals (parts per billion). No significant concentrations of ammonia, nitrite, nitrate, phosphate, or other organics (thank you zebra muscles). My tapwater is pretty clean and to be honest, I could probably get away with using it, my only concern is the silicates. I really don't want to fuel a possible algae bloom... Also, I keep some very sensetive corals and clams whose reaction to sulphates is unknown, and I really don't want to find out if it's bad. Combine that with the last fact that my water comes from and goes right back to the great lakes water system, and I've made the decision to use RO water.
Were I keeping a fish only tank, I can tell you for certain that I would not bother with it. I've met plenty of folks in my local club (all same tapwater as me) that have kept FO tanks for decades on tapwater, including one who has fish that are well over a decade old living in tapwater.
Just my personal oppinions, take em or leave em
