Lost a fish in QT

[mark]

Lost a 1.5" Potters after 18 days in what I thought was going to be a foolproof QT (converted 20G refugium, ~10lb LR with CSS65 skimmer). Found this morning when getting ready to do what would have been my 4th 5g change.

Was eating, looked good, active maybe a little faded in colour from when I got it. Only thing was last couple of days seems to be hiding a bit more when I was in the room. SG and temp good, NO3=0.0, all I had for NH3 test was an expired Hagen so wouldn't bet a 100% but measured zero.

Ideas?