I don't believe they will mix farm bred and wild cardinal tetras together. I am sure there are differences in the prices, and I don't believe any LFS will mix them on purpose.
Paracheirodon axelrodi has always been a very difficult species to breed, until there was some breakthrough not too long ago. It has been mentioned that many breeders are now producing the fish, probably majority are from the Czech Republic.
It is not known if the Asian farms have managed to reproduce them, at least I haven't heard of this. It is also to be noted that cost of shipping and/or regulations would be a consideration when importing fishes.
Therefore, I can't be sure if they are farm bred or wild fish, but... I do *think* that majority could still be wild fish (I could be wrong).
Some interesting reads,
See Wikipedia, Cardinal tetra, http://en.wikipedia.org/wiki/Cardinal_tetra (as of Jun. 28, 2008, 16:46 GMT).
http://www.practicalfishkeeping.co.u....php?news=1009
As for plecos, no, not all of them are wild. Hypancistrus zebra (L046) is almost certainly farm bred (export of wild L046 is banned). Some Ancistrus spp. (bristlenose) are almost certainly farm bred as well.
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