I've had heron problems too but haven't had any in the last 9 years. I've built a frame over each of my 2 koi ponds and covered the frames in heavy duty netting. I have tried many other things in the past but this is the only solution that has worked. It also keeps the ponds much cleaner as the leaves stay out of the ponds. My ponds are 12' by 16' and 10' by 10'. I've used the densest netting on the tops of the frames and less dense netting on the sides for better viewing of the fish. I can roll up the netting on the sides for better viewing when I want.

I too, have had to resort to netting this year when a Great Blue Heron kept coming around for lunch. I did see something inventive at my local koi store that I'd like to try. I asked them how they keep the Herons away when their ponds were open and uncovered and they told me to look up. Over our heads and almost invisible was a criss cross of heavy duty, transparent monofiliment line (fishing line) strung across from the roof to the trees above. You never noticed it at all, but I could see how it would not just startle any approaching bird, but discourage them from attemping to come too close ever again.

I put netting over my pond and the heron jumped down to the ground and walked under the porch and under the netting to go fishing. Argh!! To smart! So unless it actually encloses the pond I don't think it will keep out a hungry heron.