Hi leafs
I'm sorry about your loss,
but you can rest assured that you didn't cause the death of your fish because there was a bit of salt in the water.
While salt is harmful to corys and other catfish over an extended period of time, it can even be used as medicine to cure some resistant bacterial problems. It would not harm them so quickly.
Pandas are extremely difficult fish to keep, and especially to acclimate to your tank, and I am saying that from my own experience as well as that of the other members who have shared their problems with us. They can be fine at night and dead in the morning without a mark on them.
If you can get just about any other kind of cory at your lfs, they are bound to do better for you. Bronze and albino C. aeneus and peppered corys, C. paleatus, are perhaps the strongest of all and can even live in tanks where the water is cool. C. sterbai is beautiful, but needs warmer water than most of the others.
Any other cory your lfs might stock will almost certainly do better for you than the pandas. If your lfs doesn't have any other kinds, why not ask if they can get them for you? It's a sure thing that his distributor stocks a better variety.