If the well water does not have any toxins, just high mineral levels you could create a blend of RO + Well that suits the fish.
Many Rainbows thrive in hard water, but come from wild rivers that see extreme changes through the seasons, from very soft, low pH to very hard, high pH.
Melanotaeniidae and Pseudomugilidae
Here is how I would do this:
1) Research the fish. Find out the optimum GH and pH.
2) Make a few blends of Well + RO, and test GH, KH, TDS, pH. See which blend comes closest to what the fish want.
GH per research. KH will control the pH. TDS per research.
3) Starting with the blend that is pretty close, but perhaps just a bit too soft, add a small amount of whatever minerals are needed to correct the water to what the fish want.
GH booster for GH
Baking soda or potassium bicarbonate for KH. This also will raise the pH.
By starting with a blend of Well + RO you will likely not have to use anywhere near as much of the purchased materials.
Do you know if your well has both calcium and magnesium, and what these levels might be?