I agree with those that point out, that straight RO water is "not" good for fish, even those fish from soft mineral poor water.
If using RO, some tap water, or minerals should be mixed in.
Testing for conductivity, total hardness, and alkalinity could tell you haw much or how little to add.
And doing research into the water parameters your fish have evolved to live in would help.
General fish literature often lists the kind of parameters found at catch locations, like the ones below.
Note how different they each are from each other.
The one above, is for Hypselecara temporalis in S America.
Below water parameters Vieja bifasciata were caught in.
and below parameters normal for Herichthys cyanuguttatus (the Texas Cichlid)
These are all from the book American Cichlid Large Cichlids II by Staeck and Linke