A friend has a house that came with two identical water
heaters hooked up in parallel. He doesn't need all the capacity,
so we turned one of them off and turned off the water going
into it. This was several years ago. The water heater was
about 4 years old at the time, it's now 7 years old.

Given that a tank has already had some service, what would
you guys do to try to keep the spare tank available for as long
as possible?

Leave it full of water?

Drain it?

My thought was that draining it would be worse, because
allowing air in, it would rust.......