Diana, I'm using RO water with SS GH+. I have the water set at typical CRS parameters. KH 0, GH 5, TDS 150-160What is the GH of the water? STS does not seem to alter the GH. If it is naturally low, then look into the soft water fish.
If the GH is over about 5 degrees, then I would add carbonates and bring the pH closer to neutral.
Add some carbonates to the tank with Safe-T-Sorb.
The most common form is baking soda, but other sources work, too, such as potassium bicarbonate.
You could also add some coral sand to the filter (I use a nylon stocking to confine loose media).
Fish for very low GH, and pH include many of the South American Tetras, Dwarf Cichlids and cats. How about a pair of Apistos and an Oto?