That's a pretty big pH swing. Consider that each little .x = 10 times difference, not 1/10th difference. That would be my guess what the problem is, and maybe there is just too much CO2 and not enough O2 in the main tank, or at least a big enough difference from the QT to make the fish really not like the change. Look for other possible differences too, like stray electricity or some other thing in the main tank that's not in the QT.
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