"Sacha" wrote in message
...
On 2010-01-07 14:47:24 +0000, "Bob Hobden" said:



snip
Bob, we stop the pumps running in winter on the grounds that it makes the
water colder if it's being recirculated. I think that's the reason
anyway! Presumably you don't go along with this theory?
--
Sacha

Took me a while to get around to answering this.
Water becomes more dense as it becomes colder, and the dense cold water
sinks to the bottom of the pond.
However water is at its most dense at about 4C, so when it is very cold a
layer of water will form at the bottom of the pond at 4C and the water above
it will chill further, and form a seal of ice above the 4C water.
This in turn means that uless there is a prolonged very cold (sub-zero)
spell there will always be water at the bottom of a frozen pond which will
allow fish to survive.
You are supposed to have a very deep area in any fish pond where the fish
can retreat during a cold winter.

If you run a pump which pulls water up from the bottom of the pond and feeds
it through a filter above ground, then unless the filter is adequately
heated you will counteract the natural behaviour of the water and chill
everything down to freezing point.
This in turn is likely to kill your fish.

If you pull the water from near the top of the pond, unless the return flow
disturbs the bottom of the pond then you should be all right (untill your
filter and pipes freeze).

HTH

Dave R