Words are inadequate

Heron, yes, not egret. But yeah, they're a big problem with koi ponds. There are a range of products sold at places that sell acquatic plants and pond systems to deter them. My favorite was the motion detector attached to a hose for squirting anything moving at the right height (turn it off if you're having guests over).

They will eat your entire pond. Near me is Lilyponds, MD, which is pretty much a giant koi farm, and there are always herons around, looking for food. The owner of the place says that several times they've had breaches where the herons got in and ate so many koi they couldn't fly afterwards and had to be chased off the property on foot.