[Taras1]

It depends on your local water. RO removes most of the "unwanted" elements from your water. The DI is like "buffing" the water to a higher gloss. The heart of the system is the Reverse Osmosis Membrane, which selectively "pushes" water molecules throught the membrane, leaving the other larger molecules behind. Your Prefilter/Carbon Block/Sediment Filter primarily remove the chlorine, "biggest" stuff and particulate matter to prevent damage to the membrane.

I believe that the filters, 1 & 5 micron used in the "household" units are pretty much the same as you get from the "aquarium" quality brands. I'd be careful about the carbon blocks. Some carbon (even the activated carbon) contains significant amounts of phosphates so you might have to be careful.