FWIW - my tap water is at 220 ppm TDS and 8.2 PH. A little too hard. I added a five stage RO system and the water coming out measures at 10 ppm and 6.9 PH.

What drives me nuts is how long it takes to create even 5 gallons of RO water and how much water is wasted down the drain. The ad for this unit spec'ed 85 gallons per day, but it's not until you unpack it and read the instruction sheet that you see that you get 85 gallons @ 80 psi. Who has 80 psi? Mine is at 52 psi (unit has a pressure gage). This unit has a 3 gallon res. Took about three hours to fill a 5 gallon water container. Came out pretty good at first, but once the res is empty, you're down to a dribble. Yawn... 85 gallons? More like maybe 30 or 35.

So today I decided to try mixing RO water with the RO drain water (which has at least been filtered for sediment and chlorine), at about 50/50. End result was 7.2 PH and 160 ppm. Added some PH down, and good to go. I've got some cal-mag plus on hand, but don't think I need it with this mix. Considering a booster pump to up the capacity. Will see how this pans out.

These residential RO units are for filling a few glasses of water at a time or a pot for cooking. Trying to get 40 gallons out to fill a res requires advanced planning and a lot of water storage containers. There are high capacity RO units out there, but require really high water pressure (200 - 250 psi), run several thousand dollars, and use a lot of electricity.

I am finding RO water, in the quantities required, is a pain in the ass. If your water isn't too hard, I wouldn't go there.

Cav