If you're doing so many water changes with R/O without reconstituting it I'm not surprised they died, rainbows enjoy very mineralized water. IME celebese do fine in fresh water just not soft water. Just be careful about overthinking things, you know an ecosystyems resilliance comes from what's thrown at it over time, just like us. (however, if you are reconstituting it then no offense intended, I just thought I'd mention)
I've always found that copper rids us of invertabrates very easily. Ich goes first, being very small, but just be careful about your shrimp and snails : ). But copper sulf is IMO the most obvious and ubiquitous solution, just treat it right ; )