The TDS of my tap water is 510 ppm. Recently the GH was 6.0 degrees and the KH was 5.1 degrees. So there's a lot of other stuff going on in my tap water. It's not just Calcium and Magnesium. But I lived somewhere else that had phenomaly hard water. I remember it taking fifty drops of GH test reagent to get the sample to change from red to blue. So in your case it might actually be just a whole lot of Calcium and Magnesium. Simply phenomenal. I'm really tired of RO water too.