In my 6.5 years of ponding I have never had a problem with Heron's.
Saturday morning I was getting ready to eat breakfast and glanced out at
the pond. WHAT is that (I thought), not having my glasses on and not
totally awake. I opened the back door and low and behold a Heron is
standing on the deck of my pond. I screamed and he flew away. After a
count of the fish, none were missing but they were staying on the
bottom and would not come up. Later that evening at feeding time they
came back to the top and ate. Sunday morning my youngest son came up
stairs saying Dad there is a cool looking bird at the pond, Oh No, Not
again. all fish still there but its time to train the dog to hate this
bird. I was able to train him to chase the rabbits, squirrels and rats
so why not Herons.