Best to leave the water still
and use a bubbler at the top. Gives the
water a chance to go into layers.
Often the bottom layer is warmer
than the top layer.
A bigger concern is if your friend has
too many fish in his small pond. Overcrowding
causes lots of problems.
Fish just slow down in the cold, they don't
feel chilly like we do when we look down at
them in that cold water!

ka30p
http://www.geocities.com/watergarden...dors/home.html