When I first cycled my tank the first fish I got was two Panda Corys
One of those original Corys is still quite healthy .
However, the other one died a few weeks after getting them. The one that died never seemed quite right when it was alive.
Keep in mind that fish are no different than other animals in that there will be some individuals that die early for whatever reasons ...even in the wild .
There is nothing we can do about this of course, but give them a healthy environment and, hope for the best for all of them..
The red spot does like an injury that got worse, causing its death
I agree about the plastic as Corys also love to search the leaves and stems of plants for food, in addition to bottom feeding,
This is what makes them great cute little tank "cleaners"