As you know, without hard data, not much can be "known".

If it was my pond, I'd download this link, and send a sample of water in to be tested asap. Then I'd have hard data to tell the place where you bought the fish.

http://soiltesting.tamu.edu/files/waterweb1.pdf

Catfish are typically pretty bulletproof. If the other more fragile fish aren't kicking the bucket, then I'd suspect the catfish. BUT without the data, who's to know for sure?

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).