I have very hard water (TDS 500-700) and use this Watts RO filter
For fun I decided to send my RO water to Ward Labs to confirm it's doing a good job. Here's the results:
pH 6.6
Total Dissolved Solids (TDS) Est, ppm 59
Electrical Conductivity, mmho/cm 0.10
Cations / Anions, me/L 0.8 / 0.7...