Tap water is generally not clean. Not "clean" in the sense of "as you would find it falling out of the sky" kind of clean (though even that isn't really all that clean sometimes...). The water most of us get into our homes comes from some sort of treatment plant. In that treatment plant, they not only clean water of anything harmful that it may have in it, they also add things to the water to keep it safe to drink as it travels from the plant, through various pipes and such, into your home. The additives can be many and will vary not only for each treatment plant, but also potentially for each house (because every house has specific pipes and such that may impact the water quality). The two primary things that concern us reefers are generally
chlorine or
chloramines. Both of these additives are toxic to many organisms (thus why they are added) and should be removed for the best possible chance of success in a reef aquarium (which generally contain numerous delicate lifeforms - microscopic and larger). Beyond those two chemicals, however, are potentially other undesirable things - nitrate, phosphate, silica, and other "heavy metals" could all be in your tap water. Each of these things
may be okay (because their levels are low), but they also may not. These are all generally lumped into the measure of "TDS".
That's why some people can have success with tap water, while others do not. To really discover whether you could have any chance at a successful reef tank using tap water, you would want to find a recent water report for your area. These are generally available on the website of your water service company.
(For more info on the chemistry of all this, see
this article by
@Randy Holmes-Farley.)
what is a good RODI system you could suggest me for a 600 liters tank (+ around 70 liters sump)
There are several options out there -
BRS being the one I use myself. But even there, there are multiple options that vary in price and functionality. Which specific system is optimal for you (or more precisely; your water) will depend upon the findings in that water report - the worse your water, the more expensive unit you will need. And, of course, the worse your water, the more you would "need" an RO/DI unit...
Normally, other options include buying water from your LFS or another local reefer and storing it until needed, but at the tank size you are talking that's not going the be a realistic option for you, I wouldn't think.
is there a way to minimize the wasted water of the RODI system?
Yes!
The waste-water is a result of the RO membrane stage (other stages - however many you get - are just "flow through" and have no rejection water). The RO stage uses this "waste water" to carry contaminates out of the membrane, which prolongs the life of the membrane (allowing them to last up to a year - or more - instead of weeks or months). It's possible to use multiple membranes to "re-filter" the waste water. This has the effect of concentrating the contaminates even more (leaving you with more RO water). Any RO/DI unit should be able to be upgraded in this way. That upgrade can happen up-front, or in the future.
Another thing that can help with controlling waste water is ensuring that your water pressure is appropriate for the membrane(s). It's not uncommon for residential units to have water pressure lower than is ideal for RO membranes (especially if you are running two) and adding a
booster pump into the mix can result in long-term savings - not only in less water used, but also in the cost of membranes over the life of the system.
