I've kept African Malawi Haps and Peacocks for years, only recently getting out of it after a move. Our tap water is very, very hard. Ca/Mg hardness of 451ppm. TDS is 500ppm. Total hardness as CaCO3 is 1250ppm. All I would do with them is put in some Prime and they were happy.
Needless to say, on my two planted nano tanks I've had up for a few months, I use RO water.
Should I be reconstituing the RO water any for these tanks? Should I be mixing in some tap with the RO? It would not take much, I imagine. I've been using straight RO for changes and top offs to no ill effect so far. I'm only asking now as I am going to a pressurized Co2 setup on one of the tanks and taking it more seriously in terms of growing. It has some rotala rotundiflora in there now that grows like weeds but I am getting some glosso and ludwiga red after I put in my CO2.