welcome! There's always room for another Lynne here!
Your pond is huge, and I love the waterfall. You're lucky to have such a big pond right from the start - most of us have to start small and expand, expand, and then expand again, over the years. But I'm sorry you've had such an ordeal getting everything into balance. Sometime soon, though, you will be able to enjoy the pond and the fish and the plants.
I can't grow hyacinth or lettuce in my pond, either. Lilies do fine, but the others whither and yellow and die - My theory is that the water is too clean, because they live in little tubs and buckets with no filtration just fine.
Try putting a milk crate or a strawberry jar pot into the pond, to give the fish some place to hide. That way, they may be able to escape the heron's next visit.
I have put shade over my pond, and eliminated green water, by stringing wires from the fence to the house, and then getting an evergreen vine to grow on them. Of course, my pond is only 3' wide, and it's close to the house ... you'd have to figure some other way to get wires over your very large pond.
The other thing that helped was getting a good filter. Proper filtration, cleaned regularly, should help with your water quality problem. Also, maybe your area has particularly hard or soft water ??