Blue herons are beautiful and clever birds. In fact, many people love blue herons so much that they will travel great distances to observe and photograph them. However, if you love the fish in your ornamental pond, then the odds are good that you have no great love for blue herons, which will happily munch a pond literally to death if you do not take steps to prevent them. Fortunately, while it will require some dedication on your part, keeping your fish safe from blue herons is not particularly complicated, and can be accomplished with little to no financial outlay in most cases.
Make your pond 3 feet deep or more in all locations. Blue herons are wading birds, so if the water is too deep to wade in, they will not be able to eat your fish.
Visit your pond at irregular intervals. If you are at work all day, then return at 5:30 and frighten the heron off in the evenings, it will quickly determine that you are not around to defend your fish in the afternoon and head into your yard for lunch every day until all your fish are gone. If you cannot come by at odd times, hire a neighborhood kid or ask a neighbor to stop by so the bird cannot develop a habit of fishing your pond at a given time each day.
Set up a plastic blue heron in your pond. Except during mating season, when blue herons do tend to seek each other out, these birds are solitary hunters. If it looks like another heron is already fishing in your pond, other herons will avoid the area. In northern hemispheres they breed from March to May, and in southern hemispheres from November to April.
Create protective layers of rocks in your pond. If you cannot alter the bottom of your pond to make it deeper, create layers of decorative rocks with lots of space under them and in the layers to provide space for the fish to hide. This will help them avoid the heron and, if you can frighten the bird away enough times before the fish have time to forget it is there, you may succeed in persuading the heron to give up all together.
Aim a sprinkler over your pond. The moving water will irritate the bird and may discourage it from trying to fish in your pond. It also may agitate your fish and make them more likely to hide.