I ran a tap water reef successfully for a while. A short while, then all hell broke loose. You see in a reef nutrients are in short supply, every single thing is in competition for these nutrients. As soon as some phosphate or any other nutrient comes into the water column something snatches it up. Your tank is essentially a collection of chemical sponges. Just like a regular sponge that you use to wash dishes, they can only hold so much and then they can't hold any more.
As you use tap water in your reef you won't
see any negative effects. This is due to the contaminants, nutrients and copper being sucked up by your rocks, sand and corals. After 6 months or a year they will be saturated with nutrients and that's when all hell will break loose. Your tank will become overrun with algae and cyanobacteria. You'll then purchase an RO/DI filter and a phosban reactor and maybe another reactor to run carbon in, in an attempt to get all these harmful substances out of your reef.
At this point you are at the bottom of a very long uphill battle and you will think back to this thread and kick yourself repeatedly. Just as it took 6 months or a year for your tank to suck up all these nutrients, they will continue to leach them back out into your water for the next 6 months. Think back to 7th grade bio, osmosis. You now need to run all manner of extra equipment to grab those nutrients the second they hit the water so that they don't prolong your "cleansing" the damage caused by tap water.
For example,. two weeks ago I was too lazy to make some fresh RO/DI for my
auto top off reservoir. Instead of getting some distilled water or making RO/DI I just tossed a few pitchers of cold tap water into my reservoir. Within days I had a cyanobacteria outbreak! I couldn't believe it. Just two pitchers of tap water and I'm still fighting the cyanobacteria. It still blows my mind that such a small amount of tap water would have such a noticeable effect on my reef. The thought of using 70 gallons of the stuff after seeing what 4 quarts can do is INSANITY!
TRUST US, my RO/DI was the single best piece of equipment that I've purchased. This is 100% necessary and you should not consider doing a reef without one. It is going to cost you more time and money than you can imagine to try it without one.