So you say tap water is no good? Hmm..

[Randy Holmes-Farley]

RO/DI works perfectly well, maybe even better, without softening of hard water. What does happen is that the RO membrane gets deposits on it that render it less than effective over time.

FWIW, Spectrapure says the max hardness that they recommend is 170 ppm, so yours may be fine.

http://www.spectrapure.com/SYSTEM_BREAKDOWN.htm

I do not know of a good way around that problem, aside from softening the water or using the membrane until it's output is too low.

Perhaps others have better suggestions...