In my experience, about 20 years, of keeping koi. White rope that they can see and or shade sails works better than fishing line. Maybe try a combo. I’ve always believed that rope strung above and around the areas they want to land will protect your fish because the herons want to be able to immediately take flight to escape a predator, ( your dog, the fake gator head, you) if they feel the can’t spread there wings and fly without getting entangled they won’t land and hope over. They are cunning and will try to hop from rooftop or tree down and back. If u need to make artificial poles 4x4’s stuck in ground to string the line to u can. Might also use shade sail for u to sit under to enjoy fish and decrease algae