Gill Passman wrote:

I'm actually leaning towards the planned pond being occupied by
Shubunkins/Comets - the reasoning behind this being, that for now, I'm
not convinced that I would be able to build a large enough pond to house
koi.

Gill, there's no such thing as a pond in England that can't support "Koi or
Goldfish" - if you want to get into some of the fancier goldfish, it might
get dicey, but comets are going to be fine. There are good reasons to go
deep when raising koi, but it's not for the temperature.

Living in an area where we can get heavy frosts I need to carefully
consider temperature drops in the winter (although not to the extent of
those in even colder climates) - this winter has actually been very mild
but I can't count on this. Obviously the pond needs to be dug to a depth
below the frost line to prevent the whole thing from freezing solid.

Where's your frost line? In Ottawa, Ontario, one of our regulars did fine
with an 18" deep pond. The frost line there is well below 3' & weeks
of -20C temperatures are expected most winters. Koi & comets did fine in
my Ontario pond (which did reach 5') with 2' of ice on it.

From recent observations of the reservoir for my water feature coupled
with the depth of my old pond and how far that iced up I'm pretty sure
that I can get the depth to avoid a total freeze up.....but I am now
curious how low the temperature can drop at the bottom of the pond
without it adversely affecting the fish...

If it's liquid, they can survive. If it freezes solid, there are some
(possibly apocryphal) stories of goldfish surviving. Far more important
than the temperature is getting some oxygen in their when there's ice on
the surface for extended periods.
--
derek