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Help, what is this killing my Cardinal Tetras?

I bought 10 Jumbo Cardinal Tetras just yesturday. One died within an hour of being put in the tank, and he was swimming strange immediately. Two were dead by morning. Now two more look like their tails are disappearing. So strange, it looks like they are being eaten from the tail up. I have nothing else in the tank but a BP Pleco. I can return these guys, which I think I'll do. But I'm worried about infecting others. Should I wait to see if all the others get this (I believe there is about 4 left that look fine)? Is this contagious? Does anyone know what this is? Here's a pic. It looks like it's eating them. I need to know what to do! Yikes.

Thanks

Quote:

Originally Posted by GwenInNM (Post 729271)

Update. They're all dead. I'm going to do a WC and need to know how long I should wait to add any new fish. BTW, my water parameters are 0 ammonia, 0 nitrites and 5ppm nitrates. My tank has live plants and a sand bottom. Thanks for any help on what that was and where to go from here.

Gwen