Thanks Stephen. I found a wealth of information about RO, TDS, kH, pH on Koiphen Archives!
I also tumbled on these impressive Koi from Mike Snaden's ponds. He talked about RO water for his pond in this clip between 25 min 45 sec to 28 min 40 sec:
pH ~7 and KH below 1 degree (less than 17ppm)
https://www.youtube.com/watch?v=foLLJQVlGXA