I have never actually heard of corys being harmed by any substrate.
I would love to hear at least one story!
Please no stories about Corydoras being hamed.
They are my favorite type of fish. I also think that poor health is the cause of catfish barbels being in a sad state. Healthy aquarium = healthy inhabitants. Right? My co-workers laughed at me recently when I told them about the nitrogen cycle and the absolute nessessity of water changes with large bioloads like they were boasting about. My co-workers don't have aquariums anymore...
"I'm Wilfred Brimley, check your nitrates, check them often." (Disclaimer: I am not in actuality Wilfred Brimley.)
Sorry for the mini-rant, back to the OP.