Bob and Joyce -
The tank can freeze without a problem since the tank itself is flexible and can bend to accommodate the expanding water. And if you run the pump to the point where it is sucking air, that should ensure that the pump, and the pipe between the tank and the pump, is clear. So no worries there. I don't think we have ever had a report of a nearly-empty fresh water tank breaking from freeze.
The real problem with leaving water in the tank is the fact that crud will probably grow in the stagnant water, as RedtailCruiser mentions. Going through the standard bleach sanitizing procedure will kill the crud, at least for a while. But once it has grown, I don't know of a way to get the dead moss, algae, etc out of the tank without cutting an opening in the top or sidewall.
Bill
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