Pond owners have a number of issues to deal with when it comes to keeping their fish safe and healthy. Aside from parasites, diseases, inclement weather and cannibalism, birds remain one of the easier threats to quell. Herons don't hesitate to walk up to your pond's edge and devour a koi or goldfish if the opportunity presents itself.
Dig stakes into the ground all the way around the perimeter of your pond 30 cm up the bank. Make sure they're sticking up at least 20 cm out of the ground. String your fishing line through the stakes ensuring the line is taut with no slack or weak points. Herons will not fly directly into the water and do not like stepping over obstacles.
Install a motion-activated sprinkler and attach it to your garden hose. When the device detects movement, it quickly sprays a three-second burst of water so intruding herons will quickly leave the area and hopefully avoid it in the future. Most use very little water and even work at night.
Place a fake heron by the pond's edge near the shallowest area of the pond where herons are most likely to target your fish. Most herons, unless it's mating season, do not like fishing with a friend and will stay away as long as your "scarecrow" is present.
Although it's a little more involved, consider digging one side of the pond deeper and increasing the slope of the bank. This not only allows your fish a hiding spot, but prevents large birds like herons from sauntering up to the water's edge.
You can also install bird netting across the top of your pond to discourage herons and other unwelcome visitors.