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Are you sure the fin is ripped? I would be surprised that a plastic bag could do such damage. Plecos tend to stay in sheltered areas--they are notorious for hiding, not to be seen for days. If you are worried about the pleco, place him in a QT
tank (if you have one) so you can monitor the damaged fin. If you do not have a separate tank, watch the pleco closely. What other fish are in your tank and how big is your tank?