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My best guess is he injured himself and now it has fungused. Best I can tell from the picture. I would do extra PWCs to keep the water clean and Jungle sells a good fungus medicine.
I appreciate the help Rich...though the information unfortunately didn't end up helping. I suspected he'd be dead by the time I got home...and he was. As was one of the other GBRs. On the bright side the 3rd GBR is alive and well and looked pretty good.