I bought the four stage a few weeks ago. I tested my water going into the unit and it is about 235 tds, coming out of the ro unit its 4 tds and then coming out of the di unit it its zero. The only thing i have noticed its the di resin its changing color faster then i would have expected. I have only made about 60 gallons of water and two inches of resin has already turned brown.
Its that normal for the amount of usage?
I do weekly water changes and make about 20 gallons of RO/DI water each week. In the eight or so months that I've been using it I have changed the resin twice. The resin will change color but all you have to be concerned with is that the reading continues to read 0.00 on the output.
Happy Reefing !