If the pond has pretty much a constant level then I would assume that it is some how getting replenished by ground water somewhere, or run off......?
I agree that pump size wouldn't make a dent in this volume of water... check out
http://www.wlimproducts.com/ and/or
http://www.oasepumps.com/ for something with more punch - laugna is pretty limited and wouldn't be my first choice. Adding a water fall and/or stream feature I think would also be better than a fountain - you might even consider the addition of a few diffusers(
http://www.vertexwaterfeatures.com/wn1000.html ). Plantings would greatly help..... floating and potted/planted - such as rushes/grasses, iris's, hyacinths/water lettuce/water celery and possibly a few water lilies(but the depth is an issue, they shouldn't be more than 4ft deep at the crown) - look around you're area for native water loving plants - I really think this pond is wonderful and should stay looking as natural as possible.
*having seen this pond under snow* It's quite large and I would love to see it in the summer time. BTW Judy and your wife are quite right, drivin' on it wouldn't be a good thing...
I don't see a problem with fish over wintering in this pond - the depth itself is a grand plus - the bottom should be atleast 4-8 degrees warmer than the surface - and with a very low strocking density there may not even be a need to bother with keeping the suface open(especially if there is a natural water source coming in somewhere below the surface) .... but I wouldn't say that's a definate with out knowing more about the system as a whole. As far as what stock to put in it - well it's large enough to have a variety - you might want to consider some sort of "preditor" fish just to keep down the goldfish and or koi population - I wouldn't want to be trying to cull in a pond that size(although it's do-able).
I would like to hear a little more about this
lake
TJ
