That is exactly what I did with my discus pairs. THey
started around 175 or so hardness on a TDS-1 (from
ThreeGuysAquatics got a great price). I used straight
RO to do water changes until I got the hardness in
line. THe relationship with RO and tap mixing seems
strange. For instance, water at 175 changed 50% with
straight RO and the hardness did not get halved. Not
gonna figure out the chemistry just wanted you to know
what to expect.