I know this is about bare bottom tanks, but go ahead and put sand in your QT's. Here's a recent thread by Humblefish on the topic:
Go ahead and use sand in QT
I've never liked the bare bottom QT look, and have noticed how some fish seem uncomfortable in that environment. Especially sand burrowing wrasses, gobies and other fish who utilize sand as part of their daily routine. I've also seen tangs, angels, etc. fight their reflection on the bottom...
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