The general rule is 3' deep in at least part of the pond. I am in central North Carolina and it can freeze solid so one could stand on the pond in some years here. I only have comets, but I haven't lost any. My water level is slightly lower than I designed so I am probably a little under 3 feet and it is still OK.

I'm in zone 6A and have a 10'x8' pond that is 18" deep with a 30" deep "hole". It has never frozen solid and the only casualty in winter was a green frog.

My pond is only about 2-1/2 feet deep, but it is very small and has a small surface area (~ 4' x 5'), so that is probably partly why it has never come close to freezing solid. I run the submersible pump all winter with an open outlet, so the water circulates bottom to top and there is always at least a small open area on the surface.

As deep as you reasonably can dig is good, not only for protection from winter cold, but protection from summer heat and predators too. It's amazing how much cooler the pond will be than the air on a hot July day, and that is definitely a good thing for the goldies!

Also make sure, if the area is low and tends to collect water, that the rim of the pond will always be above water. My pond is a rigid plastic basin, and it has floated when it wasn't very full and rainwater collected underneath. When it is full it displaces all the rainwater. Your new place probably has sandy soil, unlike my solid clay, so this might never be an issue.

Do 3 1/2 feet at least.
Both for summer and winter climate conditions..

This post was edited by cliff_and_joann on Fri, Feb 15, 13 at 14:06

Check for any city/county regulations as far as fencing for deeper ponds.