My water is really hard too. I can't remember my gh measurement off hand, but kh is up around 18. Again, most fish will adapt. All of my tanks are on my tap water and the only fish I've tried to keep that I truly believe were a casualty of the water were cardinals - which are known to be especially sensitive to water like mine. But I do keep rams, neons, lemon tetras, diamon tetras, angels, plecos, several types of blue-eyed rainbows, guppies, swordtails, blue velvet shrimp, cories, etc. without issue.
Now other minerals that might be in well water could be a different story, I don't honestly know as that's not something I've dealt with. I'm on rural water, but it still goes through a rural water company/treatment vs. an actual well on my property.
I can't offer advice on an RO system as I don't use one.