water contains no dissolved minerals. This means you are not only removing the impurities you dont want, but also the minerals your tank needs. So you need to add these back if you are doing water changes with just RO
Did the issues with your fish start before or after you switched to RO
water for your water changes?
isnt something i do, so i cant really advise further than the broad concept. Maybe you should start a new thread and get specific advice on this. I know its common to do a 50/50 mix of RO
/tap water so you get the minerals from the tap water side or there are products like seachem equilibrium that you add to RO