[Pier Pressure]

I have been reading about a few deaths in the hobby lately and I am wondering about something.

If a coral dies - do you leave the skeleton in the tank or remove it? What are the pros/cons of each option?

If a fish dies - and I have had this happen - and goes missing - how come the dead body does not ruin the water chemistry? I know it would in freshwater which I have a lot more experience with.

People seem to have trouble with livestock simply "disappearing" in saltwater and I am wondering why it does not foul up the systems.

Thanks for any replies!