Hi Molotov
I would really recommend getting a full RO/DI unit. If you don't want to be bothered with the DI stage, just leave it empty. The unit will then run as an RO only unit, but if you ever decide that you want the DI you can always add it back.
Calgary water in my neighborhood tests between 150 and 180 ppm. I have an inline TDS meter right off the water supply and test regularly. Water coming out of my RO before the DI tests around 5-10 ppm after 10 minutes of operation. Water tests 0ppm after the DI chamber.
There are many people that believe you will be fine with just using tap water. I am not one of those people. In my opinion, the path to a stable system is to control as many water parameters as possible. Starting with pure water is somewhat essential to this philosophy. Without using RO/DI water, if your tank is not stable you will be second guessing yourself. Tap water simply introduces another unknown that most people prefer not to deal with.
Perhaps I am biased as we sell RO and RO/DI filters, but I can honestly say that I've been using RO/DI on my tanks for as long as I've been in this hobby and I personally feel that it is essential.
Here's what we carry.
http://www.conceptaquatics.ca/index....s-filters.html
We're in Calgary so pickup would work best.
Thanks
Brad