David,
I tried looking up the minimum depth in pub 590 and it looks like 8 to 10 feet is recommended for your area.
By far the cheapest way to add depth is to raise the dam height, assuming it is an embankment pond.
It is not only far cheaper, but it also does not require pumping out the existing pond and killing the occupants.
If you have a suitable location, raise the dam to the height you need for the additional depth.
