I tend to agree with Darrel. It seems that something in the tank is raising the hardness. This is usually something in the substrate or a decorative rock. My R/O unit is a 90 gpd, 4-stage system, but I only use 3 stages (particulate, carbon, & R/O; I see no reason to use a D/I unit, but I have naturally soft water). I suggest, with your hard water, that you get and use a back-flush accessory for your R/O membrane. It will extend its life considerably.