You may need to choose another substrate for them and switch the substrate out.
Is the CO2 new? Any chances that they're getting too much in tank? You may need to temporarily stop the CO2 to see if you can resolve the death issue.
It's good that you are switching to RO water. Are you using GH minerals added or mixing with tap? If mixing with tap, can you use RO water only with GH minerals? Maybe something changed in tap and that's causing deaths.
You also haven't mentioned what you are feeding...