I haven't had that problem.
I have read posts here that state the best way to keep heron out is to hang a fishing net up over the pond. It has to be high enough so they can't stick their beek through and stab the fish.
Hopefully others with that experience will chime in to give you a personal perspective.
they only work for a very short time as you need to constantly be moving it around. A motion sensor attached to a hose works better but even that is not 100% proof. A net is the only thing I've read about that will give you peace of mind. Search the threads for heron protection and you'll read what others have gone through.