Do you test your water params?
It would likely reduce deaths.
My test kit is due for a refill, at the moment so I can only test for nitrites and ammonia, which were both showing as fine. I found a pot of test strips that came with my last order of dechlorinator (i know they border on useless) which test for Nitrate and it was showing 5 (ish) which is roughly what my tap puts out anyway.
They had a 2 hour acclimation too, all I can conclude is possible stress from being netted out of the big tank