Check out chixlub(robs) juruense catfish. He's going to do the same thing. Run rodi and bypass a little tap so he doesn't have to remineralize it.
I'd contact an rodi specialist and let them spec a unit for you. Then you will probably have to cut that with tap water. I know the discus folks have a diy remineralize recipe that's cost effective.
These are my initial thoughts maybe someone with the same water issues will chime in