Hey blitzen25bm,
Where in SD are you? I live in temecula and work on friars road RIGHT next to the Q. have you ever been to the fallbrook koi farm? that's where I usually buy all of my koi.
First of all I feel your pain, I've lost countless koi to the GBH. over the course of my ~15 years with ponds, I've completely restocked my pond 3 times (4 - if you count the time I left the hose on overnight

). since adding the fishing line little over 2 years ago, I have only seen the GBH from afar on rooftops or on the fenceline, but never in or near the pond.
I used fishing line run from fence to fence, left to right. then ran more from the roof of my house to my fence (which is on top of a slope) at about 5' intervals. the line from the roof to the house is higher than the other and
lifts the otherset of fishing line up about 2' creating what looks almost like a wireframe dome over my yard. it isn't perfect, and there are gaps, but these predators usually wont enter an area that it feels it won't be able to escape quickly if needed. and with wing spans of up to 15 feet, the approximattly 5'X5' grid forces the heron to have to squeeze into gaps.
as a last resort, I do run fishing line at the water surface as well. I create a grid that sections the pond into smaller sections. I say last resort because supposedly once these birds are in the water, they
will not step over obticles. This allows the fish in the pond to get away from the bird while it is "trapped" in an area that fish aren't occupying. luckily I haven't had to test my theory yet.
HTH