I have a large hand dug pond in my backyard. 12'x24'x3-1/2' deep with a waterfall. All of it is landscaped. You can see pictures of it on:
For the last three years, I've had major heron problems. Some of my fish are eight years old, two feet long, and have had babies. I lost most of my goldfish, lots of my 6-inch baby koi and some koi that were 18-22 inches. They didn't eat the large ones, just beached them and poked holes in them. One heron would wait outside my fenced-in yard and walk around till we left. It would sit either in a tree or on the garage roof to wait.
Don't believe that they are territorial and will only hunt alone. If there are plenty of fish, they will share - at the same time! Morning, noon and/or night. Mine start around 5am and continue until around 8pm.
I tried to put a leaf net over the top, but they stood on the net and ripped holes in it to get the fish. (I now use a built in pool net in the winter) The heron speared the fish from outside the net and killed them without being able to get them out of the pond.
Sometimes, they flew directly into the pond and stood on my floating plant containers or on the koi protection tunnels.
Tin pans, loud noises, dogs, motion sensor scarecrows, decoys (owls, herons, ducks, alligators) - all failed. They still came back for easy snacking.
This is what worked! Use the metal fence posts that you use for gardening to keep out rabbits. I think they all come in green. I used the 3 foot ones. String either fishing line, the thick kind. (I used the XXX spider braided line and doubled it) At each post, Knot it off so when the little buggers try to get through, it won't break all the lines. (You can also use floral wire) Make sure your fence is 1 foot away from the pond edges because of their long necks. If you have a waterfall - do it through this too. The herons will climb onto the waterfall looking for a way in. Criss Cross over the top of the fence so they don't try to fly into the pond. You may want to put a gate somewhere so you can do pond maintenance.
This fence does not destroy the looks of your pond or view. the fishing line is almost invisible. It's high enough for pond plants, jumping fish, and frogs to get in and out of. It also allows the song birds to still bathe in the waterfall. You can still put a net over the top of it for the fall without having to cut your marginals back.
I keep my fencing up all year to save my fish. Good Luck.