Also, secondary infections are VERY common with velvet and can kill just as quickly as the parasites themselves. That looks to be what happened here.
The fins are definitely a sign of infection, and the velvet will have heavily infested the gills as already mentioned.
I would suggest giving the CB a freshwater dip followed by an antiseptic bath, and adding kanaplex to the QT as a preventative. I wouldn’t wait for the CB to show signs, considering they shared a QT.
See humbles emergency velvet protocol in post #2 here:
Velvet (Amyloodinium ocellatum)
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