Had the 20g been up and running for a while or did you just set it up for Qt'ing recently? Do you know if the QT tank was cycled already? Or were you reading ammonia and nitrites in the QT tank? If there is any of each (ammonia or nitrites) that could spell trouble for the fish. Aggressive water changes will help dilute it but its still there unless the tank has sufficient bacteria from a cycle.
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