Heron Decoys are widely available to prevent fish loss. Heron like to hunt and fish alone and will not stop by a pond that already has a heron in it.

The decoy has to be visible and you will probably need to move it around a bit once a week or so. (My wife and I owned a wholesale tropical fish company and later a retail pet shop. Koi and pond fish have also been in our backyard pond for two decades.)

It also helps if your pond is deeper than 24", giving the fish a deeper place to get away from the predators. And of course having a decent amount of plant cover also helps. There are also fake koi decoys you can buy. The idea is that the herons will always go for the easy catch and while doing so warn the real fish to hide.

Herons are pretty easy to deal with, but kingfishers can really clean out a pond too. Kingfishers seem much more persistent and harder to simply scare away.
Don

 
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