An RODI system really makes running a reef aquarium much easier. You do not have to go to the LFS and by RODI and mixed seawater. You can make your own and you control the quality instead of hoping that someone else does their job right.
The first consideration is does your tap water have chloramines or chlorine? Chlorine or chloramines will attack your RO filter and quickly degrade it. A good carbon block will absorb chlorines. You need a much more robust one for chloramines.
The next consideration is what is the TDS of your tap water? If it is low, less than 80 tds, even a modest system should do well for you. If it is moderate, 80-200 tds, you probably want a system that is a bit more upscale. And if it is over 200 tds, you want to have an RO filter or system that will get the tds down low enough so you don't chew through piles of DI resin removing that final tds.
My tap water is chlorinated so a standard carbon block system does fine. My tds is about 130, my RO filter gets the tds down to 2.
If you water pressure is low, less than say 50 psi, you may need to have a booster pump to get the water pressure high enough for your RO filter to work properly. My pressure is 65 psi and my standard system does OK.
I run a 6 stage Bulk Reef Supply system. It has a basic filter to remove particulates from the water, it has two carbon blocks, it has an RO filter, it has a flush system to flush the RO membrane, and I have two canisters of DI resin hooked in series. When the first depletes, I remove it. I put the second one in the first position, repack the first one and put it in the second's position. That way I fully deplete the DI resin in the canister.
I have a large aquarium. My standard water change is 40 gallons. And I go through about 40 gallons of RO water every two weeks. So I must make about 2000 gallons per year. I change the carbon blocks an sediment filter about every 6 months. I am on my original RO filter after about 3 years of use.
