Lost a new cory :(

Yesterday I purchased three Peppered cory cats (corydoras paleatus). They're a bit thin, but eating - this seems the norm, whenever I bring new fish home they're always very hungry - but they seem to be in great shape. All the fish in the tank at the store were active and healthy looking.
This morning all three were swimming around the tank looking fine, then one started acting strange - breathing hard and hanging onto the cork wall of the tank, not coming down to eat breakfast with the other two - and 2 hours later he died.
It's been about 18 hours since I brought them home, could the little guy have died from the stress of being shipped?
I have a book which lists this species of cory as a 'robust species' that's happy living in a wide range of pH and temperatures, so they seemed perfect (I've had some bad luck in the past with pygmys dying suddenly with no signs of disease, and wanted something a little less delicate that would be happy at a pH of 7.4 living with livebearers and loaches).
All the tank parameters are stable (see my tank specs) and all the other fish in the tank look just fine. Any ideas?

I did a lot of research before I built the back wall, stainless steel and cork are both safe (cork is actually really common for back walls) to use in freshwater aquariums - cork does break down over time like driftwood/bogwood, but like wood it's quite safe.
I used stainless because I know some plastics/rubbers get brittle over time and some leach chemicals into the water, and the steel's part of a wall - it's pretty much permanent, and needed to be strong, with holes small enough that fry couldn't get through. It doesn't rust like galvanized steel (which is what the screws that hold slate onto the bottom of driftwood and other decorations are made of so it must be fairly safe too, even though it does rust and will disintigrate after being in the aquarium for a couple years). The tank's been up and running for six months or so (maybe longer), and none of the fish have shown any ill effects (the pygmys that I lost were in my 45g, I moved the one that's left into the 20g recently for the same reason I moved the neons - they were all starting to look like snacks to my angelfish). I've raised fry in the 20g and figure they're quite sensitive to chemical and other problems with water, so if anything in the tank was poisonous they wouldn't be so healthy.
However... I have been wrong before, and if I'm accidentally hurting my fish someone please let me know!