A properly installed and fuctioning RO/DI unit will remove most of the minerals and many other contaminants often found in some tap water.
The degree to which these compounds are removed depends on the brand/specifications of the unit, and vary somewhat among manufacturers.
I use a Kent MAX60HiS 5 stage 60 gallon per day unit myself and according to info on their website and instruction sheet - it will remove 99.9% of tap water contaminants, providing lab grade water.
To answer your question - yes - most RO/DI units will strip everything out of the tap water - leaving it very pure - but in doing so I find that I need to restore this water with buffers to add back some of the carbonates, bicarbonates and other salts. The pH is 7.0 as well.
Good thing is I am comfortable knowing the water is free of crap that could cause problems in my tanks. The bad thing is that I must add additional compounds, buffers/salts to the RO/DI water used for top offs and mixing salt. Otherwise it will pull down the pH in the tank, and makes it hard to maintain correct levels of alkalinity and calcium.
It's a trade off for sure, but my tap water here is so unpredictable - I chose to invest in one.
In my tap water, I've detected nitrates and low level phosphates, as well as a normal pH of between 8.5 to 9.0 - no kidding.
You'd be surprised what the pH is of your own tapwater, if you haven't tested it yet.
Use a fresh water pH kit obviously.
Most good quality RO/DI units will remove chlorine as well.
HTH
