Hello; I agree that WC is something you can do that is pretty much a safe thing to do. Only one concern comes to mind about WC, that being if the pH between the tank water and the source water are much different. This can occur when it has been a while since a WC. In such a case the WC should be done at a smaller % in stages for a few days.
I also agree that water parameters are good information to know.
The condition of a dead fish is not as useful as the condition of an afflicted fish while it is still living. If any of the other living fish show symptoms please post that.