This is a slightly edited post from an earlier thread about winterising, it is in reference to storing pumps over winter.

Posted by horton 6 b Ontario. (My Page) on Fri, Nov 2, 07 at 8:51

Magnetic pumps do not require to be kept in water over the winter, that is usually a way of storing oil filled pumps.
The seals in an oil filled pump should not be allowed to dry out.

If your pump is the magnetic type and you decide to take it out of the pond to store it or not, it should be taken apart at the end of the pond season and cleaned out.
Cleaning the pump's impeller and rotor to remove the gunk that has accumulated in there over the summer, is advised by the pump manufacturers.

This accumulated gunk can set up like glue over the winter and can/will inhibit the pump's impeller/rotor from rotating when you plug the pump in next spring.
The gunk setting up inside the pump's housing is usually the cause of magnetic type pumps burning out in the spring.
Even if you have an oil filled pump, stored in pond or out of pond in a bucket of water to keep the seals pliable,just cleaning out the impeller housing is a worthwhile job.

To clean a mag pump, first remove the impeller cover/housing [usually four screws hold it on] and ease the cover away from the pump body.
Carefully pull the impeller out and away from the body of the pump, straight out, as the shafts are made of a ceramic material and can snap if bent or twisted.
You will feel a slight resistance, this is just residual magnetism in the rotor, keep pulling the impeller/rotor assembly out.
Once you have the impeller/rotor out from the pump cavity, wash all the gunk off/out using an old[or new if you like] toothbrush or small bottle brush.
Dry all the parts with paper towel, put a little dab of vaseline on the ends of the rotor shaft and re-assemble.
Your pump is ready for next season.

There is more information on winterising a pond in the FAQ link below.

http://faq.gardenweb.com/faq/lists/ponds/2003010923017442.html
DRH [David] who contributed the winterising article, along with Ronaye and Steve, uses an aquarium air pump and air-stone bubbler in his pond in Vermont [Zone 4] all winter long to keep a vent hole open in the ice.
I use a small pond pump to keep the water surface moving over a large area, so it does no freeze.
There are various methods/devices used for venting ponds.
Water or air pump driven aerators, electrically powered de-icers, stock-tank heaters, home made light bulb de-icers, all ranging it wattage's from 60 to 1500.
Some do the job some don't.
Installing the type of device you are planning to use, should be done NOW.
Don't wait till the pond is covered with ice and snow, it is a hateful job trying to vent an iced over pond.

Having a back-up, like Sandy has, is a good idea also.
These de-icers can fail, just like anything else, so having something else to take over is prudent thinking.

Whatever method you choose, having it in place and functioning, prior to a freeze up, gives the device a better chance of doing it's job.
"Horton"