Herring is a fish. Heron is a bird.

I've heard most people say that you actually need two decoys, since herons are only territorial when nesting.

I wouldn't trust any kind of a decoy to deter herons. By my way of thinking, it's just announcing to the predator population that there is food there. And once they find your pond, they will empty it without precautions.

The scarecrow will probably work for a while, until it discovers that the jet of water doesn't hurt it. Loud, barking dogs are also good deterrents, since herons don't like to dine when they're being harrassed. However, the net is really the only fool-proof method of protecting the fish, provided its properly secured.

A true and rather humorous story: A friend of mine found a mannequin and set her out beside the pond with the heron decoy lying on its side at her feet, giving the illusion that she had killed it. In this case, it worked. The heron did a couple of fly-bys, and hasn't been seen since!

I've almost come to the conclusion that there is no one way to keep the herons or other predators away entirely. What seems to repel most only makes others curious or more determined. Maybe they are just really hungry. I haven't seen a heron except flying by, since putting up a pair of decoys.

That said, yesterday I looked up from the computer to see a lovely hawk perched on the back of a chair, watching the pond and the fishies. Guess the fall net goes up this week.

It was a mad Herring, with teeth like the "\ /". Thank you waterwoman, my bad. I've been checking the net everyday and I think its working. I've also heard on youtube clips of placing movement sensors hooked up to spot lights and a noise maker.
I feel bad for my fish, its getting cold here in Pennsylvania. The time of year I would pay closer attention to feeding for the winter closing. My fish are so terrified, I can't even see them in the caves.