Do you have any surface agitation or bubbler in the stored RO water?
If not dissolved oxygen may be low, which will cause a quick reaction by fish.
Yes, I have the RO water sitting in containers. So it’s prob low oxygen!
Yes, I do! TDS after water change was 180ish.
The actual value is not as important as the rate of change. Large sudden changes can stress the fish.
Yes, that is prob the problem! Any way to fix this problem?