Interesting thread and eagerly waiting for more input. So far, I'm
gathering the consensus is you want a bit of circulation at the bottom?
Until last year I've used the method of a couple of airstones slightly
below the surface, which has worked with no loss of fish. Last year, due
to laziness on my part, I left an Oase Pond30 Aerator at the bottom,
which in my mind gave some (minimal) water movement and another opening
in the ice, as well as the usual two airstones (3 or 4" from the
surface), again with no fish loss. I'd really like to see more
discussion on this.

deanna (zone4, SE Idaho)

RichToyBox wrote:

If the water becomes warmer than 39 degrees, it will rise, and once the pond
is above 39, any cooling of the surface causes the water to sink until the
whole pond is 39 degrees and then there is no flow. If the winters are
extremely cold, and long, then the soil under the liner could be cooled to
below 39, but it will be getting heat from further down in the ground.
During this time, the bottom water will be below 39, but it will still be
the densest water in the pond and the circulation will cease. The shallower
the pond, the quicker the soil under the pond will cool, due to its closer
proximity to the cold source. For shallow ponds it could be possible that
the circulation is so little for so long that toxic gasses could become
saturated and oxygen not diffuse due to lack of currents and harm the fish,
I guess. The use of the air stone will cause a very slight turnover, with
almost no current and should be beneficial.


jan wrote:
In my case, that would mean when switching to my 300 gph pump in winter,
continuing to pull from both bottom drain and skimmer (making the skimmer
almost useless at that flow). I'm really thinking to stay with skimmer
only, especially with a spic & span bottom for the koi to sit on, but
another alternative would be an airstone on the bottom, something we're
always telling others not to do.
Some days this hobby is SO confusing. ~ jan
See my ponds thru the seasons and/or my filter design:
http://users.owt.com/jjspond/